Chronotype-Dependent Changes in Sleep Habits Associated with Dim Light Melatonin Onset in the Antarctic Summer.

Ana Silva,Álvaro Díaz,Cecilia Casaravilla,Diego Simón,Bruno Pannunzio,Bettina Tassino

doi:10.3390/clockssleep1030029

Abstract

Dim light melatonin onset (DLMO) is the most reliable measure of human central circadian timing. Its modulation by light exposure and chronotype has been scarcely approached. We evaluated the impact of light changes on the interaction between melatonin, sleep, and chronotype in university students (n = 12) between the Antarctic summer (10 days) and the autumn equinox in Montevideo, Uruguay (10 days). Circadian preferences were tested by validated questionnaires. A Morningness–Eveningness Questionnaire average value (47 ± 8.01) was used to separate late and early participants. Daylight exposure (measured by actimetry) was significantly higher in Antarctica versus Montevideo in both sensitive time windows (the morning phase-advancing and the evening phase-delaying). Melatonin was measured in hourly saliva samples (18–24 h) collected in dim light conditions (<30 lx) during the last night of each study period. Early and late participants were exposed to similar amounts of light in both sites and time windows, but only early participants were significantly more exposed during the late evening in Antarctica. Late participants advanced their DLMO with no changes in sleep onset time in Antarctica, while early participants delayed their DLMO and sleep onset time. This different susceptibility to respond to light may be explained by a subtle difference in evening light exposure between chronotypes.

Highlights

The biological clock has evolved in all living beings to adapt their functioning to a predictable light–dark cycling environment
Considering that in the Antarctic summer crew members are more exposed to light in both light-sensitive time windows with opposite effects on circadian shifts, these results indicate that, on average, the morning window is more important in resetting the circadian phase
Though participants with earlier and later circadian preferences were exposed to a similar amount of light in each site and in both time windows, in Antarctica, late chronotypes advanced their Dim light melatonin onset (DLMO) with no changes in sleep onset time and early chronotypes delayed their DLMO and sleep onset time

Summary

Introduction

The biological clock has evolved in all living beings to adapt their functioning to a predictable light–dark cycling environment. It is indisputable that modern urban life has affected human light exposure by increasing it during nighttime (due to electric light and screens) and decreasing it during daytime (due to indoor work in enclosed buildings). It is a matter of current debate how the human clock copes with these quite recent changes in environmental light, by which the reliable day–night natural cycle has been distorted [6,7,8]. The dim light melatonin onset (DLMO, the start of the evening rise of melatonin in plasma or saliva) is the best proxy of the functioning of the circadian clock in humans [10,16]. It is clear that seasonal changes in day length affect the duration and phase of the nocturnal melatonin pulse [23,24]

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Results

Conclusion