Alerting and Circadian Effects of Short-Wavelength vs. Long-Wavelength Narrow-Bandwidth Light during a Simulated Night Shift

Round 1

Reviewer 1 Report

The authors investigated how narrowband blue and red light could affect alertness, DLMO, mood, performance and visual comfort over extended light exposures during a simulated night work environment. The experimental setup comprised of ceiling mounted LED luminaires targeting a high photon density for each light source. Task performance and visual comfort was improved under blue light condition. The study also reported reduced sleepiness and greater circadian phase shift for the blue light compared to the red light.

The results presented in the current study replicate those reported extensively by past studies. The manuscript is well structured and easy to follow. However, lack of novel insights into the field of non-visual effects of light and low precision measurements at the eye dampen the excitement for an otherwise well drafted study.

Major comments:

1. Page 2, Line 90: As reported by the authors as well, several past studies have indeed reported improved alertness, performance, greater circadian effectiveness, for blue light sources. So I fail to grasp the novelty behind the hypotheses.
2. Page 4, Line 158: Were the study participants seated during the breaks? Were they allowed to walk within the laboratory space? Given the calibration of the ceiling mounted light sources was performed assuming a vertical plane, were the participants instructed to look straight ahead or were they looking down most of the times to undertake activities such as reading? Were any spot measurements undertaken in the gaze direction of the study participants to verify the actual light levels delivered at the eye? A cosine drop off could substantially lower the effective corneal light level
3. Line 441: The novel goal of the study to replicate previous findings at a higher light level (which inherently requires high precision for stimulus delivered), is in direct conflict with the secondary goal of a naturalistic setting involving free gaze for study participants

 

Minor comments:

1. Page 2, Line 64: Figueiro et al. 2009 also included LED light sources, and at light levels comparable with those employed in the current study (https://pubmed.ncbi.nlm.nih.gov/19712442/)
2. Page 2, Line 85: It will be of interest to the reader if light levels on the horizontal plane for the 2 spectra are also mentioned
3. Page 4, Line 158: Please elaborate on “other tests and questionnaires”
4. Page 4, Line 178: Please revise the table 1 to reflect the alpha-opic irradiances in accordance with the latest CIE S 026/E:2018 guidelines (http://cie.co.at/publications/cie-system-metrology-optical-radiation-iprgc-influenced-responses-light-0)
5. Page 8, Line 315: Was any statistical analyses performed to detect outliers?
6. Line 467: Need to also acknowledge Bullough et al. 2014 who have shown that spectral sensitivity for brightness perception is dominated by short wavelength photoreceptors (https://www.tandfonline.com/doi/full/10.1080/15502724.2013.827516?src=recsys) which could explain greater brightness perception for the blue light source from the current study

Author Response

Thank you for reviewing our manuscript, and for your valuable comments. We have now addressed point by point the comments, and we believe that the manuscript is much improved. Please see our responses, as well as the corresponding manuscript changes, below in red.

 

The authors investigated how narrowband blue and red light could affect alertness, DLMO, mood, performance and visual comfort over extended light exposures during a simulated night work environment. The experimental setup comprised of ceiling mounted LED luminaires targeting a high photon density for each light source. Task performance and visual comfort was improved under blue light condition. The study also reported reduced sleepiness and greater circadian phase shift for the blue light compared to the red light.

The results presented in the current study replicate those reported extensively by past studies. The manuscript is well structured and easy to follow. However, lack of novel insights into the field of non-visual effects of light and low precision measurements at the eye dampen the excitement for an otherwise well drafted study.

 

Response: We are pleased that the reviewer has noted that the study was well drafted. We acknowledge that the results to some extent replicate those reported previously. However, we do believe that the study shows some novelty, especially concerning the administration of narrow-bandwidth lighting, which was administered via standard ceiling mounted luminaires during a simulated night shift. We also employed higher (arguably with higher ecological validity) photon density light than most previous studies, although the low precision measurements of the light stimulus at the eye warrants further validation of these findings. We have now tried to better emphasize the novelty in terms of light administration by rewriting and adding some information to the Introduction and also to other parts of the manuscript according to the responses given to the specific comments (see comments below).

 

We agree that the light measurements had low precision in terms of light reaching the eye. As the study investigated the effects of narrow-bandwidth lighting during a simulated night shift, we did not strictly control the posture of participants throughout the study. However, we believe that the reported light levels represent the approximate light exposure at eye level during most of the time. Several changes have been made to clarify this (see comments below).

 

Major comments:

1. Page 2, Line 90: As reported by the authors as well, several past studies have indeed reported improved alertness, performance, greater circadian effectiveness, for blue light sources. So I fail to grasp the novelty behind the hypotheses.

 

Response major point 1: We agree that the novelty may not have been clear in the hypotheses. We have now made some changes and added information to the hypotheses in order to highlight the use of ceiling mounted luminaires in a naturalistic setting which represent the main novelty.

 

Line 114 (125): ‘We hypothesized that high photon density short-wavelength narrow-bandwidth light, administered by standard ceiling mounted LED-luminaires, would lead to better alertness, mood and performance during the night shift, compared to a night shift with long-wavelength narrow-bandwidth light. We also hypothesized that the night shift with short-wavelength light would lead to a larger phase delay of the circadian rhythm, compared to a night shift with long-wavelength light. In addition, we investigated participants’ subjective evaluation of the lighting conditions, as well as visual comfort during the night shift.’

 

2. Page 4, Line 158: Were the study participants seated during the breaks? Were they allowed to walk within the laboratory space? Given the calibration of the ceiling mounted light sources was performed assuming a vertical plane, were the participants instructed to look straight ahead or were they looking down most of the times to undertake activities such as reading? Were any spot measurements undertaken in the gaze direction of the study participants to verify the actual light levels delivered at the eye? A cosine drop off could substantially lower the effective corneal light level

 

Response major point 2: We acknowledge the need for clarifying the issues concerning participants’ seating and gaze. The participants remained seated at their designated workplace throughout the laboratory study. Participants’ posture and gaze were not controlled, as we wanted the behavior to mimic behavior in an ecologically valid setting. Most of the time however, participants were looking straight ahead to complete tests administered via computers. However, during breaks (usually 10-15 min) participants were allowed to engage in reading, hence they were allowed to look down. The light measurements were made at approximately eye level and in the direction of gaze while seated. We did not include additional light measurements during the night shifts. We have now added information concerning these issues into the paper.

 

Line 192 (208): ‘Participants had several short breaks (usually 10¬–15 min) allowing quiet activities (e.g. reading and talking). Participants remained seated at their designated desk space for most of the time during the whole shift, except for toilet breaks, for which they had to walk through a dimly lit hallway.’

 

Line 217 (231): ‘Note that participants’ posture and gaze direction were not strictly controlled (except when engaged in the performance tasks). The light levels thus represent the approximate light exposure at eye level during most of the time in the laboratory.’

 

3. Line 441: The novel goal of the study to replicate previous findings at a higher light level (which inherently requires high precision for stimulus delivered), is in direct conflict with the secondary goal of a naturalistic setting involving free gaze for study participants

 

Response major point 3: We understand this point made by the reviewer. We have now toned down the notion regarding replication at higher light levels.

 

Line 487 (507): ‘The current results thus add to previous findings showing that short-wavelength narrow-bandwidth light, administered by standard ceiling mounted LED-luminaires in a naturalistic setting, also elicits alerting and performance enhancing responses, compared to long-wavelength narrow-bandwidth light. In addition, the results indicate that these effects can be achieved using higher photon densities than previously reported.’

 

We have now also made some changes towards the end of the introduction by rephrasing and deleting some text and references.

 

Line 107 (117): ‘Similar photon density (~ 2.8 x 10¹⁴ photons/cm²/s) was used for both the short-wavelength (λ_max = 455 nm) and the long-wavelength (λ_max = 625 nm) narrow-bandwidth light, with photopic illuminances of 60.8 lx and 195.9 lx, respectively.’

 

Minor comments:

1. Page 2, Line 64: Figueiro et al. 2009 also included LED light sources, and at light levels comparable with those employed in the current study (https://pubmed.ncbi.nlm.nih.gov/19712442/)

 

Response minor point 1: We understand that the sentence needs to be more specific. We referred to Figueiro et al. 2009 earlier and are aware of their use of LED light boxes and illuminance of 40 lx. Thus, we have now made some minor changes to the paper by specifying that we are referring to the few studies using ceiling mounted LED-luminaires. In addition, after an editorial note, we have now also added a reference to a recent study from our own group using similar LED-based lighting for administering light during simulated night work.

 

Line 89 (94): ‘Thus, light exposures previously administered by specialized lighting set-ups, can now be administered via standard ceiling mounted LED-luminaires applicable for illumination of workplaces. However, only a few recent studies have used such LED-based lighting during simulated night work [33,40,41].’

 

2. Page 2, Line 85: It will be of interest to the reader if light levels on the horizontal plane for the 2 spectra are also mentioned

 

Response minor point 2: We understand the interest of the horizontal light levels, however such measurements were unfortunately not conducted during the night shifts.

 

3. Page 4, Line 158: Please elaborate on “other tests and questionnaires”

 

Response minor point 3: We understand the need for this information and have now added this in the text.

 

Line 187 (204): ‘Between the main test bouts, other tests and questionnaires were administered, such as a pegboard test, working memory, reversal learning, numerosity discrimination, and a Go/No-go test, as well as a questionnaire on evaluation of moral issues. In this paper we report results from the main test bouts only, in addition to visual comfort and evaluation of the lighting conditions. The protocol was the same during both night shifts.’

 

4. Page 4, Line 178: Please revise the table 1 to reflect the alpha-opic irradiances in accordance with the latest CIE S 026/E:2018 guidelines (http://cie.co.at/publications/cie-system-metrology-optical-radiation-iprgc-influenced-responses-light-0)

 

Response minor point 4: We agree with the reviewer and have now revised the light measurements using the CIE S 026 Toolbox—version 1.049. Thus, Table 1 has now been slightly modified and the melanopic EDI, and alpha-opic irradiances are now presented. Accordingly, relevant numbers in other parts of the paper have also been changed.

 

We have now also changed the reference [35] from Lucas et al. 2014 to CIE S 026/E:2018.

 

5. Page 8, Line 315: Was any statistical analyses performed to detect outliers?

 

Response minor point 5: No, although we did notice some extreme values for some of the measures, e.g. PVT lapses, which may indicate that some participants were not able (or did not bother) to perform optimally. However, we retained all the measurements as the results (conclusions) did not seem to be affected by this.

 

6. Line 467: Need to also acknowledge Bullough et al. 2014 who have shown that spectral sensitivity for brightness perception is dominated by short wavelength photoreceptors (https://www.tandfonline.com/doi/full/10.1080/15502724.2013.827516?src=recsys) which could explain greater brightness perception for the blue light source from the current study

 

Response minor point 6: We thank the reviewer for pointing out this reference. We have now added this information and the reference to the text.

 

Line 533 (555): ‘Also, it has been shown that brightness perception has a short-wavelength spectral sensitivity that increases with increasing light levels [61], which could explain the greater brightness perception of the short wavelength light in our study.’

Reviewer 2 Report

Please see the attached document for my comments.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

The physical properties of light (wavelength and photon density), but also the phase of the circadian cycle predetermine its physiological and psychological effects. The manuscript presents some interesting and relatively new findings with possible practical applications. Relevant study limitations (factors of individual light sensitivity, chronotype, sex, and age) are properly considered in the discussion.

Novel findings are related to LED-based light conditions during night work. As one would expect, the results demonstrated the predominant efficacy of monochromatic blue light compared to red light in relation to sleepiness, visual comfort, task performance, and melatonin phase. For mood, though, the effects of blue light were not found beneficial since positive mood was reduced.

Practically results suppose that blue light is proved more useful to keep shift-workers staying alert by night, and these effects can be achieved using standard LED-based lighting set-ups.

On the other hand, it once again confirms that during night time the impact of blue light (e.g. monochromatic blue vs. monochromatic red) on the circadian system is more prominent, thereby a considerable disruptive effect upon the circadian system can be exerted when such a factor is being acted during nocturnal phase continuously for sufficiently long (the threshold for such duration is yet to be determined) period of time.

Author Response

 

Thank you for reviewing our manuscript. See our response below in red.

 

The physical properties of light (wavelength and photon density), but also the phase of the circadian cycle predetermine its physiological and psychological effects. The manuscript presents some interesting and relatively new findings with possible practical applications. Relevant study limitations (factors of individual light sensitivity, chronotype, sex, and age) are properly considered in the discussion.

 

Novel findings are related to LED-based light conditions during night work. As one would expect, the results demonstrated the predominant efficacy of monochromatic blue light compared to red light in relation to sleepiness, visual comfort, task performance, and melatonin phase. For mood, though, the effects of blue light were not found beneficial since positive mood was reduced.

 

Practically results suppose that blue light is proved more useful to keep shift-workers staying alert by night, and these effects can be achieved using standard LED-based lighting set-ups.

 

On the other hand, it once again confirms that during night time the impact of blue light (e.g. monochromatic blue vs. monochromatic red) on the circadian system is more prominent, thereby a considerable disruptive effect upon the circadian system can be exerted when such a factor is being acted during nocturnal phase continuously for sufficiently long (the threshold for such duration is yet to be determined) period of time.

 

Response: We thank the reviewer for the nice summary and interpretation of the main findings of our paper, as well as for the positive remarks.

Reviewer 4 Report

In general, a well written paper and an interesting study that adds to the puzzle of how light impacts humans beyond illumination.

The following suggestions could improve the manuscript.

Next to minor issues, I have the following major issues.

The authors state that this study is unique because it uses ambient lighting with a relative high photon density under simulated night shift conditions.

For the “uses ambient lighting with a relative high photon density” the following remarks:

1. I agree with the authors that the applied lighting conditions in the study are unique, but this does not automatically mean that unique is the synonym of ‘relevant’. Having said this, I am convinced that more studies like this, all following a similar study protocol, will contribute to the ultimate question what light recipe we can recommend for night shift workers. Two literature studies on shift workers (Slanger, T.E.; Gross, J.V.; Pinger, A.; Morfeld, P.; Bellinger, M.; Duhme, A.L.; Reichardt Ortega, R.A.; Costa, G.; Driscoll, T.R.; Foster, R.G.; et al. Person-directed, non-pharmacological interventions for sleepiness at work and sleep disturbances caused by shift work. Cochrane Database Syst. Rev. 2016, 2016 and Neil-Sztramko, S.E.; Pahwa, M.; Demers, P.A.; Gotay, C.C. Health-related interventions among night shift workers: A critical review of the literature. J. Work Environ. Health 2014, 40, 543–556) give a nice overview of studies related to night workers and some more specifically to effects of light. I miss in introduction and discussion the unique positions of this manuscript related to earlier studies. In a recently published paper clocks & sleeps (Aarts, M. P. J.; Hartmeyer, S. L.; Morsink, K.; Kort, H. S. M.; de Kort, Y. A. W. Can Special Light Glasses Reduce Sleepiness and Improve Sleep of Nightshift Workers? A Placebo-Controlled Explorative Field Study. Clocks & Sleep 2020, 2 (2), 225–245) an analyses of field studies on the impact of light during nightshifts is given and can be useful for this manuscript as well.

 

For the “simulated night shift conditions” the following remarks:

1. The simulated night shift conditions seem a bit away from reality. As far as I understood from the manuscript is that the participants were not performing any task (except the vigilance tests). Additionally, one of the major problems in people working nightshifts, is that you don’t always want to phase shift your clock, especially if you will be working only one or two consecutive night shifts.
2. What is also missing in the study design as well as in the discussion, is that the sleep and recovery after a shift is not considered at all. It is one thing to have people more alert during the night, but another, and maybe an even more important one on the long term, is to support sleep as well. Look at the work of Souman et al. (Souman, J. L.; Borra, T.; de Goijer, I.; Schlangen, L. J. M.; Vlaskamp, B. N. S.; Lucassen, M. P. Spectral Tuning of White Light Allows for Strong Reduction in Melatonin Suppression without Changing Illumination Level or Color Temperature. J. Biol. Rhythms 2018, 33 (4), 420–431) who supplied white (LED) lighting with and without additional blue light (between 450-500 nm) and demonstrated that it was possible to minimize the disturbance in melatonin production without impacting alertness. In another study (Aarts, M. P. J.; Hartmeyer, S. L.; Morsink, K.; Kort, H. S. M.; de Kort, Y. A. W. Can Special Light Glasses Reduce Sleepiness and Improve Sleep of Nightshift Workers? A Placebo-Controlled Explorative Field Study. Clocks & Sleep 2020, 2 (2), 225–245) the effect of blue light emitting glasses for night shift workers was assessed over a complete nightshift-period of 3-5 days, addressing the use of these glasses but also the potential impact on sleep and alertness under a real situation. They also recommended a personal light regime instead of ambient lighting conditions, since the ‘light recipe’ (time, timing, spectral composition, intensity, duration) depending on the number of consecutive nights and position in row (e.g. first night is different than the last night) of each individual. This of course is not relevant when night shift workers would all work according the same rotation schedule. This issue should therefore be addressed.
3. So the fact that there isn’t so much literature on blue light during the might for shift-workers is that the additional negative side-effects sometimes are too severe. Also look at the literature and discussion on (breast)cancer due to light during the night.
4. My suggestion therefore would be to add the before mentioned to the discussion, with the additional literature that relates more closely to the topic and specific issues related to night workers.

 

Minor issues

Introduction

• The authors claim that the use of ambient, monochromatic light was not applied in earlier studies (line 55-58) due to technological restrictions while I am quite sure that the reason was a methodological one.

Materials and methods

• Since the impact of light beyond illumination, or Non-Image Forming effects of light, like phase shifting and alertness, is related to several aspects of which intensity, spectral distribution, duration and timing are considered in this study. The aspects not addressed are directionality, and moreover light history. It is only discussed that since the study took place on such high latitude during winter, the daytime light exposure would not have impacted the results. I would expect, since the intention is to compare the effects of two light situations during nighttime, and we know that light history might have an effect, this would have been monitored as well.
• It would have been very interesting to compare the results to a control group under polychromatic light conditions. Additionally, since monochromatic light negatively impacts the color rendering qualities and therefore visual performance (think of use of colors).
• About the study design, see earlier comments (Major issues) on addressing sleep as outcome measure.
• Photopic illuminance values in table 1 are not identical than described in line 85.
• The term alpha-opic lx (table 1) is incorrect. In a recent publication from CIE (CIE System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light; International Standard CIE S 026/E2018; CIE: Paris, France, 2018; Volume 32) a standard is defined on how to express and communicate optical radiation (light) with respect to ipRGC-influenced responses, e.g. in alpha-opic equivalent daylight (D65) illuminance. I suggest to re-calculate the photon density values used in this study, according to the standards.

Results

For me the results of figure 5 are not fully comprehensible. While under the red light conditions, the phase markers remain stable between baseline and final, except for 3 participants indicating a difference of more than 1.5 hours (later), so also a strong shifting effect. For the blue light conditions, the phase markers show much more deviations, interesting enough also going shifting to earlier.

• Are the (3) shifters under the red light conditions also the shifters under the blue light conditions?
• Are the people who shift back under blue light conditions, the same as the once under red light conditions? This could mean that not some people might be more sensitive to light than others.

 

Discussion

Miss a discussion on the limitations and the relevance for night shift workers. See earlier comment in Major issues.

See remarks on Major issues, specifically on the relevance of this study.

Author Response

Thank you for reviewing our manuscript, and for your valuable comments. We have now addressed the comments, and we believe that the manuscript is much improved after this revision. Please see our responses, as well as the corresponding manuscript changes, below in red.

 

In general, a well written paper and an interesting study that adds to the puzzle of how light impacts humans beyond illumination.

 

The following suggestions could improve the manuscript.

 

Next to minor issues, I have the following major issues.

 

The authors state that this study is unique because it uses ambient lighting with a relative high photon density under simulated night shift conditions.

 

For the “uses ambient lighting with a relative high photon density” the following remarks:

 

1. I agree with the authors that the applied lighting conditions in the study are unique, but this does not automatically mean that unique is the synonym of ‘relevant’. Having said this, I am convinced that more studies like this, all following a similar study protocol, will contribute to the ultimate question what light recipe we can recommend for night shift workers.

 

Two literature studies on shift workers (Slanger, T.E.; Gross, J.V.; Pinger, A.; Morfeld, P.; Bellinger, M.; Duhme, A.L.; Reichardt Ortega, R.A.; Costa, G.; Driscoll, T.R.; Foster, R.G.; et al. Person-directed, non-pharmacological interventions for sleepiness at work and sleep disturbances caused by shift work. Cochrane Database Syst. Rev. 2016, 2016 and Neil-Sztramko, S.E.; Pahwa, M.; Demers, P.A.; Gotay, C.C. Health-related interventions among night shift workers: A critical review of the literature. J. Work Environ. Health 2014, 40, 543–556) give a nice overview of studies related to night workers and some more specifically to effects of light. I miss in introduction and discussion the unique positions of this manuscript related to earlier studies.

 

In a recently published paper clocks & sleeps (Aarts, M. P. J.; Hartmeyer, S. L.; Morsink, K.; Kort, H. S. M.; de Kort, Y. A. W. Can Special Light Glasses Reduce Sleepiness and Improve Sleep of Nightshift Workers? A Placebo-Controlled Explorative Field Study. Clocks & Sleep 2020, 2 (2), 225–245) an analyses of field studies on the impact of light during nightshifts is given and can be useful for this manuscript as well.

Response: We acknowledge the reviewer’s comments.

 

We have now made changes to the manuscript and have added paragraphs to the Introduction, expanding on the background in terms of night shift studies, among others by including the references suggested by the reviewer. We have also included additional references as requested by the editor and other reviewers. The most relevant additions regarding this comment are:

 

Line 67 (70): ‘While the use of such specialized lighting set-ups allows for well controlled laboratory trials, the suitability in real-life settings may be limited.

Night work has consistently been associated with alertness and performance deterioration [24,25], and light interventions have the potential to counter these immediate effects of night work, both by its acute alerting properties [26] and via circadian phase shifting [27]. However, reviews of interventions to reduce the negative impact of night work (also chronic health effects) have indicated that definite conclusions on the beneficial effects of light interventions during night work cannot yet be drawn [28,29]. Recently, it was also noted that although most field studies indicate some beneficial effect of light during night work, methodological issues and diversity preclude conclusion about appropriate light schedules for night shift workers [30].

Due to nonvisual responses being sensitive to short-wavelength light, there has been an interest in employing short-wavelength enriched (i.e. blue-enriched) white light as a countermeasure against some of the negative impacts of night shift work. As such, recent studies have suggested beneficial effects of blue-enriched light on nocturnal alertness and performance [31,32,33]. On the other hand, an issue with the use of light interventions during night work, especially short-wavelength light, is the potential negative effects associated with light at night, e.g. melatonin suppression has been suggested as a mechanism for the increased risk of cancer among night shift workers [34,35]. Thus, studies have also investigated short-wavelength depleted/attenuated white light during simulated night shifts [36,37,38]. These studies have indicated that such lighting can reduce melatonin suppression and phase shifting of the melatonin rhythm, without having a negative impact on alertness and performance.’

 

We have now also added a paragraph to the Discussion, regarding the relevance for night workers.

 

Line 580 (603): ‘The present study demonstrates a novel use of ceiling mounted LED-luminaires for administering narrow-bandwidth light conditions during simulated night work. Furthermore, we employed relatively high photon densities of light, that may be realistic for real-life work situations. However, the practical relevance for night workers remains debatable. Ambient narrow-bandwidth lighting alters visibility and color rendering (i.e. color appearance of the surroundings), hence for many workplaces the narrow-bandwidth lighting employed in the present study may not be feasible. The novel way of administering the narrow-bandwidth light conditions in the current study, however offers new opportunities for illumination that need further investigation in terms of feasibility for specific workplaces and settings. A concern with short-wavelength light in particular, relates to the potential negative impact of light at night [34,35], e.g. melatonin suppression and circadian disturbance. The present results indicate strong phase shifting effects of the short-wavelength light. While such effects may be practical for permanent night workers, they may at the same time be regarded as unwanted effects for rotating night workers. Another consideration, not assessed in the present paper, is the impact of light interventions on sleep and recovery after night work, as sleep disturbances are one of the main issues with night work [24,25]. Thus, there is a need to consider which effects of light are most desired in specific work situations and settings in particular for night workers.’

 

For the “simulated night shift conditions” the following remarks:

 

1. The simulated night shift conditions seem a bit away from reality. As far as I understood from the manuscript is that the participants were not performing any task (except the vigilance tests). Additionally, one of the major problems in people working nightshifts, is that you don’t always want to phase shift your clock, especially if you will be working only one or two consecutive night shifts.

 

Response: We have added more information to the methods stating that other tasks were also performed during the simulated night shifts (also requested by another reviewer). We agree that the simulated shifts are not that realistic in terms of similarity to work tasks in many occupations, but simulated shift work studies are essential/valuable to test different lighting conditions.

 

Line 187 (204): ‘Between the main test bouts, other tests and questionnaires were administered, such as a pegboard test, working memory, reversal learning, numerosity discrimination, and a Go/No-go test, as well as a questionnaire on evaluation of moral issues. In this paper we report results from the main test bouts only, in addition to visual comfort and evaluation of the lighting conditions. The protocol was the same during both night shifts.’

 

We have now also added, to a new paragraph in the Discussion, some discussion regarding that phase shifting may not be desirable in some situations.

 

Line 590 (613): ‘The present results indicate strong phase shifting effects of the short-wavelength light. While such effects may be practical for permanent night workers, they may at the same time be regarded as unwanted effects for rotating night workers. Another consideration, not assessed in the present paper, is the impact of light interventions on sleep and recovery after night work, as sleep disturbances are one of the main issues with night work [24,25]. Thus, there is a need to consider which effects of light are most desired in specific work situations and settings in particular for night workers.’

 

2. What is also missing in the study design as well as in the discussion, is that the sleep and recovery after a shift is not considered at all. It is one thing to have people more alert during the night, but another, and maybe an even more important one on the long term, is to support sleep as well. Look at the work of Souman et al. (Souman, J. L.; Borra, T.; de Goijer, I.; Schlangen, L. J. M.; Vlaskamp, B. N. S.; Lucassen, M. P. Spectral Tuning of White Light Allows for Strong Reduction in Melatonin Suppression without Changing Illumination Level or Color Temperature. J. Biol. Rhythms 2018, 33 (4), 420–431) who supplied white (LED) lighting with and without additional blue light (between 450-500 nm) and demonstrated that it was possible to minimize the disturbance in melatonin production without impacting alertness.

 

In another study (Aarts, M. P. J.; Hartmeyer, S. L.; Morsink, K.; Kort, H. S. M.; de Kort, Y. A. W. Can Special Light Glasses Reduce Sleepiness and Improve Sleep of Nightshift Workers? A Placebo-Controlled Explorative Field Study. Clocks & Sleep 2020, 2 (2), 225–245) the effect of blue light emitting glasses for night shift workers was assessed over a complete nightshift-period of 3-5 days, addressing the use of these glasses but also the potential impact on sleep and alertness under a real situation. They also recommended a personal light regime instead of ambient lighting conditions, since the ‘light recipe’ (time, timing, spectral composition, intensity, duration) depending on the number of consecutive nights and position in row (e.g. first night is different than the last night) of each individual. This of course is not relevant when night shift workers would all work according the same rotation schedule. This issue should therefore be addressed.

Response: As sleep and recovery were beyond the aims of the present paper and since the paper is lengthy and already presents much data we have not focused on sleep. However, we acknowledge the importance of addressing this topic and we have added some discussion on this to the Discussion. In addition, the Souman et al. reference (ref. 38) has now been included in the new paragraph added to the Introduction.

 

Line 83 (86): ‘Thus, studies have also investigated short-wavelength depleted/attenuated white light during simulated night shifts [36,37,38]. These studies have indicated that such lighting can reduce melatonin suppression and phase shifting of the melatonin rhythm, without having a negative impact on alertness and performance.’

 

Line 592 (615): ‘Another consideration, not assessed in the present paper, is the impact of light interventions on sleep and recovery after night work, as sleep disturbances are one of the main issues with night work [24,25].’

 

We have now discussed and suggested individually tailored light interventions to be investigated in the discussion. We have also added the reference Arts et al. 2020 (ref. 30).

 

Line 603 (626): ‘Furthermore, we did not employ individually tailored light exposure, as suggested by a recent study [30]. Due to the inter-individual differences in circadian phase timing, the fixed work schedule, and uniformly lit work environment, the light exposure occurred at different circadian times for different participants. Hence, individually tailored light exposure would likely lead to even larger phase shifting effects than observed.’

 

Line 616 (645): ‘Future studies should investigate the possibility of providing individually tailored light exposure, using standard ceiling mounted LED-luminaires, e.g. by programming luminaires to provide favorable light exposure at individual workplaces. Furthermore, there is a need for more studies to assess the amount of melatonin suppression throughout the night shift under different light conditions.’

 

3. So the fact that there isn’t so much literature on blue light during the might for shift-workers is that the additional negative side-effects sometimes are too severe. Also look at the literature and discussion on (breast)cancer due to light during the night.

 

4. My suggestion therefore would be to add the before mentioned to the discussion, with the additional literature that relates more closely to the topic and specific issues related to night workers.

Response: We appreciate the reviewer’s comments and have included several points on the possible negative effects of light at night as noted in some of the previous responses. See especially the first response where we addressed these issues.

 

Minor issues

 

Introduction

 

The authors claim that the use of ambient, monochromatic light was not applied in earlier studies (line 55-58) due to technological restrictions while I am quite sure that the reason was a methodological one.

Response introduction: We do understand the comment and have now added a sentence in order to clarify this point.

 

Line 67 (70): ‘While the use of such specialized lighting set-ups allows for well controlled laboratory trials, the suitability in real-life settings may be limited.’

Materials and methods

Since the impact of light beyond illumination, or Non-Image Forming effects of light, like phase shifting and alertness, is related to several aspects of which intensity, spectral distribution, duration and timing are considered in this study. The aspects not addressed are directionality, and moreover light history. It is only discussed that since the study took place on such high latitude during winter, the daytime light exposure would not have impacted the results. I would expect, since the intention is to compare the effects of two light situations during nighttime, and we know that light history might have an effect, this would have been monitored as well.

 

It would have been very interesting to compare the results to a control group under polychromatic light conditions. Additionally, since monochromatic light negatively impacts the color rendering qualities and therefore visual performance (think of use of colors).

 

About the study design, see earlier comments (Major issues) on addressing sleep as outcome measure.

 

Photopic illuminance values in table 1 are not identical than described in line 85.

 

The term alpha-opic lx (table 1) is incorrect. In a recent publication from CIE (CIE System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light; International Standard CIE S 026/E2018; CIE: Paris, France, 2018; Volume 32) a standard is defined on how to express and communicate optical radiation (light) with respect to ipRGC-influenced responses, e.g. in alpha-opic equivalent daylight (D65) illuminance. I suggest to re-calculate the photon density values used in this study, according to the standards.

Response materials and methods: We appreciate the comments regarding directionality and light history. In terms of directionality some information has now been added to the methods.

 

Line 215 (231): ‘Note that participants’ posture and gaze direction were not strictly controlled (except when engaged in the performance tasks). The light levels thus represent the approximate light exposure at eye level during most of the time in the laboratory.’

 

We acknowledge the comment regarding prior light history. We did not thoroughly control prior light history, but light exposure was monitored by light sensors on the actigraph device. We have now added a supplementary Table A1, where we provide light exposure in the hours (18:00–22:45) preceding the night shifts. Analysis indicated no significant difference between the light conditions. We have now added a sentence regarding this to the methods section. We acknowledge in the discussion that light history may have affected the results, and we have also made some changes to this part.

 

Line 165 (181): ‘The light sensor on the Actiwatch was also used to assess light exposure in the hours preceding the night shifts (see Appendix Table A1).’

 

Line 597 (620): ‘We did not thoroughly control participants’ light exposure prior to the night shifts as has been done in previous laboratory studies. Light exposure in the hours (18:00–22:45) preceding the night shifts was monitored by the light sensor on the actigraph device, indicating no significant difference between conditions. Prior light history is known to affect nonvisual light responses including the alerting response [65], hence this may have also had an impact in the current study. However, the relatively high latitude and time of year ensured limited daylight exposure in the hours preceding the night shifts.’

 

We agree with the comment regarding the use of a control group for comparison with polychromatic light. However, we did not incorporate a night shift with polychromatic light in this study. We have now added a comment on this to the Discussion.

 

Line 497 (519): ‘We did not have an additional night shift in dim light or in standard light conditions, hence the alerting effects of our light conditions cannot be compared to dim light or standard light conditions.’

 

We have now also added text regarding the color rendering of the light conditions

 

Line 583 (606): ‘However, the practical relevance for night workers remains debatable. Ambient narrow-bandwidth lighting alters visibility and color rendering (i.e. color appearance of the surroundings), hence for many workplaces the narrow-bandwidth lighting employed in the present study may not be feasible.’

 

We have now corrected the numbers in the text to match those in the Table, Line 109 (120).

 

The lighting parameters have now been re-calculated using the CIE standard. Table 1 has now been updated accordingly, and we have added the Melanopic EDI. We have also changed the corresponding numbers in the text after this update.

 

Results

 

For me the results of figure 5 are not fully comprehensible. While under the red light conditions, the phase markers remain stable between baseline and final, except for 3 participants indicating a difference of more than 1.5 hours (later), so also a strong shifting effect. For the blue light conditions, the phase markers show much more deviations, interesting enough also going shifting to earlier.

 

Are the (3) shifters under the red light conditions also the shifters under the blue light conditions?

 

Are the people who shift back under blue light conditions, the same as the once under red light conditions? This could mean that not some people might be more sensitive to light than others.

 

Response results: We agree with the reviewer’s comments on the results in Figure 5. We have already noted in the discussion that differences in sensitivity to light may have affected the results:

 

Line 512 (535): ‘However, the individual differences in circadian responses indicate that factors other than the light are also at play, such as differences in sensitivity to light [56]. Notably, following the night shift, the melatonin rhythm of a few participants showed the opposite phase shifting response (i.e. phase advance), although they had a similar initial melatonin onset time.’

 

To answer the specific questions. There were four participants shifting more than 1.5 hours after a night shift with long-wavelength (red) light. The Table below shows the corresponding phase shift with short-wavelength (blue) light (data available in supplementary file). The Table also shows two participants that phase advanced with short-wavelength light

 

Participant	Long-wavelength light	Short-wavelength light
6	3:03 h	Not available
22	1:39 h	2:24 h
28	2:51 h	2:58 h
32	3:19 h	-0:01 h
8	0:26 h	-1:03 h
14	Not available	-0:41 h

Discussion

 

Miss a discussion on the limitations and the relevance for night shift workers. See earlier comment in Major issues.

 

See remarks on Major issues, specifically on the relevance of this study.

 

Response discussion: We agree that the relevance for shift workers needed to be more thoroughly discussed and have now added a paragraph on this to the Discussion. See the response to the first comment were the paragraph is included.

Reviewer 5 Report

This study of Erlend Sunde and colleagues is of interest and timely as the authors take a very practical approach in addressing the potential beneficial effects of blue light exposure during shift work with respect to sleepiness and task performance. Having said so the idea is not very novel and has been shown by many other groups before, but the approach is and the authors clearly state this.

I only have a few textual suggestions:

• In the abstract information is missing on what the simulated night shift involves as is some information on the type and duration of the light exposure. This would be relevant given that the practical approach the authors took is the most interesting aspect of this manuscript.
• The introduction is relatively long. I suppose that everyone knows what the benefits of LEDs are, and this may be removed. Also all the technical information on photon density may not be necessary.
• Like the introduction, also the discussion is relatively long. I would also recommend to discuss the practical implementations and limitations of their findings. For example, the phase delay induced by blue light would actually be a benefit in a permanent shift work cycle, but not very helpful in fast rotating shift work. Also it is not know if the blue light is still helpful in consecutive night shift.

Author Response

Thank you for reviewing our manuscript and for the comments. Please see our responses, as well as the corresponding manuscript changes, below in red.

This study of Erlend Sunde and colleagues is of interest and timely as the authors take a very practical approach in addressing the potential beneficial effects of blue light exposure during shift work with respect to sleepiness and task performance. Having said so the idea is not very novel and has been shown by many other groups before, but the approach is and the authors clearly state this.

I only have a few textual suggestions:

• In the abstract information is missing on what the simulated night shift involves as is some information on the type and duration of the light exposure. This would be relevant given that the practical approach the authors took is the most interesting aspect of this manuscript.
• The introduction is relatively long. I suppose that everyone knows what the benefits of LEDs are, and this may be removed. Also all the technical information on photon density may not be necessary.
• Like the introduction, also the discussion is relatively long. I would also recommend to discuss the practical implementations and limitations of their findings. For example, the phase delay induced by blue light would actually be a benefit in a permanent shift work cycle, but not very helpful in fast rotating shift work. Also it is not know if the blue light is still helpful in consecutive night shift.

Response: We thank the author for the positive feedback. We have made some changes as suggested. However, although we appreciate the recommendation to cut down the introduction, we have not been able to comply with this, due to relevant feedback from the editor and other reviewers requesting more information on several topics to be added.

 

We agree with the reviewer’s comment regarding information in the abstract and have now added information on the type and duration of the light exposure.

 

Line 23 (25): ‘…, during a simulated night shift (23:00–06:45 h), conducting cognitive performance tasks. Light conditions were administered using ceiling mounted LED-luminaires.’

 

We agree with the reviewer’s comment regarding knowledge of benefits of LEDs and have now removed some text and references in the introduction.

 

Line 89 (92): Removed text and two references.

 

We chose to retain the photon densities as we believe these will be of interest for comparison between different studies.

 

We agree with the reviewer’s comments regarding the practical use. We have now added text and references in several places regarding the practical use of similar light conditions, also following a similar request from other reviewers. The most relevant additions are provided below.

 

Line 67 (70): ‘While the use of such specialized lighting set-ups allows for well controlled laboratory trials, the suitability in real-life settings may be limited.’

 

Line 580 (603): ‘The present study demonstrates a novel use of ceiling mounted LED-luminaires for administering narrow-bandwidth light conditions during simulated night work. Furthermore, we employed relatively high photon densities of light, that may be realistic for real-life work situations. However, the practical relevance for night workers remains debatable. Ambient narrow-bandwidth lighting alters visibility and color rendering (i.e. color appearance of the surroundings), hence for many workplaces the narrow-bandwidth lighting employed in the present study may not be feasible. The novel way of administering the narrow-bandwidth light conditions in the current study, however offers new opportunities for illumination that need further investigation in terms of feasibility for specific workplaces and settings. A concern with short-wavelength light in particular, relates to the potential negative impact of light at night [34,35], e.g. melatonin suppression and circadian disturbance. The present results indicate strong phase shifting effects of the short-wavelength light. While such effects may be practical for permanent night workers, they may at the same time be regarded as unwanted effects for rotating night workers. Another consideration, not assessed in the present paper, is the impact of light interventions on sleep and recovery after night work, as sleep disturbances are one of the main issues with night work [24,25]. Thus, there is a need to consider which effects of light are most desired in specific work situations and settings in particular for night workers.’

 

Round 2

Reviewer 1 Report

I would like to thank the authors for doing a commendable job in addressing my original concerns. I believe the manuscript reads much better now and provides good ideas for subsequent research.

Reviewer 2 Report

I would like to thank the authors for responding to my concerns and addressing each of my lengthy comments.