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Weekly and seasonal variation in the circadian melatonin rhythm in humans: A response
Author(s) -
Zerbini Giulia,
Merrow Martha,
Winnebeck Eva C
Publication year - 2022
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.12777
Subject(s) - zeitgeber , circadian rhythm , melatonin , entrainment (biomusicology) , context (archaeology) , biology , daylight , chronotype , chronobiology , light cycle , seasonality , rhythm , circadian clock , ecology , neuroscience , medicine , paleontology , physics , optics
We read with interest the commentary by Skeldon and Dijk about our article “Weekly, seasonal and chronotype‐dependent variation of dim light melatonin onset.” The discussion points raised by Skeldon and Dijk are currently among the most hotly debated in human circadian science. What external factors determine human phase of entrainment? How great is the contribution of natural versus artificial light and sun time versus social time? Our intra‐individual data add to the still limited evidence from field studies in this matter. In their commentary, Skeldon and Dijk formulate two either‐or hypotheses, postulating that humans entrain either solely to the natural light‐dark cycle (sun time referenced by midday) (H 1 ) or solely to the light selected by local clock time and social constraints (H 2 ). Neither hypothesis accounts for the effect of season on human light exposure. We interpreted our findings along more complex lines, speculating that the 1‐h earlier melatonin rise in summer found in our sample is likely the combined result of daylight saving time (DST)‐induced behavioral advances and a stronger natural zeitgeber in summer (light exposure determined by social and seasonal factors, H original ). Here, we show how the criticism by Skeldon and Dijk is based on two sentences quoted out of context (misrepresenting our hypothesis as H 1 ) and that their hypothesis H 2 leaves out important seasonal components in light exposure.