Light Exposure Rhythms and Sleep Organization in Adolescents: Temporal Differences Between Weekdays and Weekends in an Actigraphic Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1.The authors correctly note that relative spectral irradiance was measured, not standardized a-opic metrics (e.g., melanopic EDI) per CIE S 026. This is a significant limitation. It is recommended to further emphasize in the discussion that all inferences regarding the "non-visual" circadian impact of light are therefore indirect, and to suggest the use of standardized metrics in future research.

2.The sample size (N=18) is small and recruited from a single region and specific school system (full-time) in Brazil, which limits the generalizability of the findings. It is recommended to discuss this more thoroughly in the "Limitations and Future Directions" section, acknowledging that the sample may not represent the broader adolescent population and suggesting larger, multi-center studies in the future.

3.The discussion cites literature well but could engage more deeply with classical theories (e.g., Pittendrigh's generalizations) to explain why basal circadian stability (e.g., IS, IV) was preserved despite significant differences in light patterns. This would strengthen the theoretical contribution of the study.

4.The captions and in-text references for figures (e.g., Figure 1, 2) could be more descriptive. For instance, the caption for Figure 1 could clarify that it shows mean temperatures at different times of day. Additionally, the frequent use of "p<0.001" in the results could be occasionally replaced with terms like "significant" for better readability.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this ambulatory monitoring study of adolescents, the investigators found deficient morning light (school lighting) on weekdays compared to weekends, but no real consequence of this in terms of circadian rhythmicity. Weekend sleep was shorter than weekday sleep. LIght spectrum exposure was homogeneous. The methods and premise are strengths. The writing is clear.

The main weakness of this thorough study of circadian rhythmicity and activity/sleep time lies in presentations in Discussion that appear discordant with Results in which the authors emphasize discussion points either in contrast to their data (which they don’t discuss in context) or emphasize points not evaluated in their study. Discussion can be substantially improved and shortened.

An example is in Discussion 3.1, they continue to discuss the possible symptomatic effects of low morning school lighting as if there were effects. The authors should note in this section that, despite the light exposure differences, no circadian consequences occurred. The first sentence in section 3.2 does state this, but some similar statements belongs somewhere in 3.1.

Another example is Discussion 3.3. The subjects, despite weekday evening light exposure, slept longer on weekdays, not shorter. The contrast with the prevailing data should be acknowledged.

In Discussion 3.5, they note that nighttime temperature nadir is higher on weekends; could this observation be to masking effects/overall increased physical activities on weekends?

The authors mention in several locations in Discussion that phase delays may gradually accumulate and thereby affect sleep-wake, but I don’t see evidence of that accumulated phase shifting demonstrated here. Omitting these points in Discussion would be my recommendation; alternatively, using the subjects’ serial activity in double-plotted actigraphy of weekday data would be able to demonstrate phase shifting. Since they have temp data, plotting the possible shifting of the temperature nadir also would help support their oft-mentioned cumulative phase delays.

Finally, if there’s no real differences between red-green-blue light exposure during the day, how would this change recommendations for circadian light mitigation such as iPhone and other device changes to “circadian safe light” modes?

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This study addresses an important topic in sleep science: systematic differences between weekdays and weekends as markers of circadian misalignment. Focusing on adolescents is especially valuable, given their distinct circadian physiology and the practical implications for education and public health. The study demonstrates methodological and analytical rigor and is grounded in a sound scientific framework.

However, several areas would benefit from strengthening. The manuscript would be improved by a more critical engagement with the key literature on social and biological time in adolescents; this will help situate the findings within current debates (e.g., social jet lag, school start times, light sensitivity during puberty). The clinical and occupational implications of the results should be expanded and more clearly articulated, particularly with respect to recommendations for schools, clinicians, and policy makers.

Additional methodological information is required to ensure reproducibility and to evaluate the validity of the findings. Please provide details on the actigraphy devices used (model), calibration procedures, data-processing software and settings, and the preparatory procedures for the selected sample. The rationale for the small sample (n = 18) should also be justified — ideally with a sample-size calculation or power analysis. In observational research, small samples limit the ability to detect effects and increase uncertainty; the manuscript should acknowledge these limitations and discuss their impact on interpretation and generalizability.

Overall, the study makes a meaningful contribution, but addressing the points above will improve transparency, contextualization, and the applicability of the findings.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The manuscript presents a valuable, ecologically valid research of how daily light exposure is organized in adolescents and how it differs between weekdays and weekends. The use of actigraphy with integrated spectral RGB sensors is a strong methodological choice, allowing objective analyses of the data on both activity and light. The authors’ focus on the temporal and spectral dimensions of light adds nuance to the field’s understanding of social-time influences on circadian regulation. Overall, the study makes a meaningful contribution and offers a solid foundation for future interventions aimed at optimizing light exposure in school settings.

On the other hand, there are several crucial issues that should be addressed:

1. Seasonal changes in light environment should be addressed: The study was conducted over a week though not simultaneously. Most likely 24-hour patterns of light exposure differed between first and last observational weeks due to seasonal photoperiodic fluctuations. It could be interesting to compare, or have photoperiod duration as co-factor in the models. Consider discussing the potential impact of the specific photoperiod (e.g., May–July in Brazil) on the observed 24-h profiles. If possible, include photoperiod length as a covariate in the ANOVA models or, in future work, compare weeks with different day lengths.

2. Within the 15–17 year range, chronotype may shift toward eveningness with increasing age. Treat age (or at least age group) as a covariate in the analyses or report stratified results. This would clarify whether the observed weekend–weekday differences are consistent across the age span.

3. Table 1 lacks mean sleep onset (or bedtime) values, which are known to shift later on weekends, especially in later chronotype adolescents. Adding this metric would provide a more complete picture of how light exposure relates to actual sleep timing.

4. Table 2. Tables should be fully self-explanatory and all abbreviations like DS − FDS, BlL, RL, GL should be explained. Regarding this Table 2, it is likely that relative decrease in received light at weekends across all spectrum at 18:00-22:00, followed by increase thereafter from 22:00 and later reflects changes in physical activity. It can be explained by homeward back from school activities at weekdays given the schedule time frames provided. Have the authors correlated light exposure vs activity by clock time? It is also likely that such patterns reflect social jet-lag phenomena, and its relative prominence may be closely linked to chronotype (habitual timing of sleep/activity schedules).

5. In this context it is a pity that the authors did not include mean time of sleep in Table 1, which, unlike other metrics of duration and quality are known to change substantially at weekends depending on chronotype but also history of light exposure and physical activity. As later timing of sleep-wake cycle on weekends in adolescents is well-documented, results must be discussed in this context too.

6. The 2 hour bins used in the figures may obscure finer temporal shifts between weekdays and weekends. Consider presenting a higher resolution (e.g., 30-minute) plot or a smoothed curve to better capture timing differences.

7. The authors did not report how many boys/girls there were. This is important as previous studies reported significant sex-related differences in many circadian endpoints, that may also include 24-h patterns of light exposure.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors were responsive to critiques. 

Author Response

Thank you for your reviewing.

Reviewer 4 Report

Comments and Suggestions for Authors

I thank the authors for the reasonable answers and accordingly improvements. I have no further comments at this time. 

Author Response

Thank you for your reviewing.