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Objectives: Light is not only crucial for vision but also influences other aspects of physiology and behavior. One well studied aspect of light is its effect on our biological clock which is also linked to sleep timing. Yet, the effect of daytime light on nighttime sleep itself has hardly been studied in humans. In our within-subject design laboratory study we examined the influence of daytime light on the subsequent nighttime sleep. We measured effects on the biological clock as well as on sleep homeostasis. Methods: Participants with mild sleep complaints were exposed to three conditions: Either to one day (9–17 h) with moderate intensity polychromatic ‘office light’, one day with high intensity polychromatic light ‘bright light’, or one day with extra blue LED light on top of the ‘office light’. The influence of this daytime light on the biological clock was measured via analysis of the melatonin rhythm and on sleep quality/homeostasis via sleep-EEG measurements. Multilevel regression analyses were used to quantify the effects on the accumulation rate of slow wave activity. Results: Compared to melatonin onset in the office light condition, the onset was earlier in the bright light condition (0.35 h, p < 0.05) and there was a trend for an earlier onset in the blue light condition (0.30 h, p = 0.07) compared to the office light condition. Sleep efficiency was higher (95.8% vs. 92.1%, p < 0.05) and the percentage of REM sleep was larger (23.3% vs. 19.7%, p < 0.05) in the bright light condition compared to the office light condition. The accumulation rate of slow wave activity during sleep, a measure of sleep pressure decay, was significantly higher in the brightas well as in the blue light condition compared to the office light condition. Conclusions: Light intensity during the day affects sleep via an effect on the clock and by influencing sleep homeostasis, with higher light intensities leading to more consolidated, deeper sleep. Funding/Disclosures: This study was financially supported by an unrestricted research grant of Philips Consumer Lifestyle B.V. Drachten, The Netherlands, developed by the legal authorities of both parties.

Supplemental material, Supplementary_information_White_light_during_daytime_does_not_improve_alertness_in_well-rested_individuals for White Light During Daytime Does Not Improve Alertness in Well-rested Individuals by Renske Lok, Tom Woelders, Marijke C. M. Gordijn, Roelof A. Hut and Domien G. M. Beersma in Journal of Biological Rhythms

Animals in seasonal environments need to fit their annual-cycle stages, such as moult and migration, in a tight schedule. Climate change affects the phenology of organisms and causes advancements in timing of these annual-cycle stages but not necessarily at the same rates. For migratory birds, this can lead to more severe or more relaxed time constraints in the time from fledging to migration, depending on the relative shifts of the different stages. We tested how a shift in hatch date, which has advanced due to climate change, impacts the organization of the birds' whole annual cycle. We experimentally advanced and delayed the hatch date of pied flycatcher chicks in the field and then measured the timing of their annual-cycle stages in a controlled laboratory environment. Hatch date affected the timing of moult and pre-migratory fattening, but not migration. Early-born birds hence had a longer time to fatten up than late-born ones; the latter reduced their interval between onset of fattening and migration to be able to migrate at the same time as the early-born birds. This difference in time constraints for early- and late-born individuals may explain why early-born offspring have a higher probability to recruit as a breeding bird. Climate change-associated advancements of avian egg-lay dates, which in turn advances hatch dates, can thus reduce the negative fitness consequences of reproducing late, thereby reducing the selection for early egg-laying migratory birds.