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Weekly, seasonal, and chronotype‐dependent variation of dim‐light melatonin onset
Author(s) -
Zerbini Giulia,
Winnebeck Eva C.,
Merrow Martha
Publication year - 2021
Publication title -
journal of pineal research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.881
H-Index - 131
eISSN - 1600-079X
pISSN - 0742-3098
DOI - 10.1111/jpi.12723
Subject(s) - zeitgeber , chronotype , entrainment (biomusicology) , melatonin , circadian rhythm , rhythm , sunset , chronobiology , sunrise , nocturnal , circadian clock , phase response curve , atmospheric sciences , environmental science , endocrinology , biology , medicine , physics , astronomy
In humans, the most important zeitgeber for entrainment is light. Laboratory studies have shown that meaningful changes in light exposure lead to phase shifts in markers of the circadian clock. In natural settings, light is a complex signal varying with external conditions and individual behaviors; nonetheless, phase of entrainment is assumed to be fairly stable. Here, we investigated the influence of season and weekly schedule (as indicators of variation in light landscapes) on phase of entrainment. Using a within‐subjects design (N = 33), we assessed dim‐light melatonin onset (DLMO) as a circadian phase marker in humans, on workdays and work‐free days, in summer (under daylight saving time) and in winter, while also estimating sleep times from actimetry. Our mixed‐model regressions show that both season and weekly structure are linked with changes in phase of entrainment and sleep. In summer, both DLMO and sleep times were about 1 hour earlier compared to winter, and sleep duration was shorter. On work‐free days, DLMO and sleep times were later, and their phase relationship differed more relative to workdays. All these effects were stronger in later chronotypes (those who habitually sleep late). Our results confirm that phase of entrainment is earlier when stronger zeitgebers are present (summer) and show that it relates to midday or midnight rather than sunrise or sunset. Additionally, they suggest that late chronotypes are capable of rapid phase shifts each week as they move between workdays and work‐free days, stimulating interesting questions about the stability of circadian phase under natural conditions.