![]() In mammals, daily rhythms in physiology and behavior are under control of a circadian pacemaker situated in the suprachiasmatic nucleus (SCN). ![]() Here we show how the criticism by Skeldon and Dijk is based on two sentences quoted out of context (misrepresenting our hypothesis as H1) and that their hypothesis H2 leaves out important seasonal components in light exposure. We interpreted our findings along more complex lines, speculating that the one‐hour 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, Horiginal). Neither hypothesis accounts for the effect of season on human light exposure. 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) (H1) or solely to the light selected by local clock time and social constraints (H2). 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 adds to the still limited evidence from field studies in this matter. The discussion points raised by Skeldon and Dijk are currently among the most hotly debated in human circadian science. We read with interest the commentary by Skeldon and Dijk about our article “Weekly, seasonal and chronotype‐dependent variation of dim light melatonin onset”. Such a misalignment induced by where people live within a time zone may affect a large share of the population, with implications for health and safety. The findings suggest that humans entrain to sun time, leading to an increasing discrepancy between social time and biological time (“circadian misalignment”) towards the west of a time zone. Regression results showed a progressive delay in MSFWe from east to west within three of the four US continental time zones (delay per degree of longitude): Eastern, 1.8 min Central, 1.2 min Mountain, 2.4 min (all p <. ![]() Longitude position was derived from state-level information (e.g., average state border outline). Chronotype was estimated based on mid-sleep time on weekends (MSFWe), a proxy that was previously shown to replicate known age and sex differences in chronotype in the ATUS. In this brief report, we aimed at estimating the relationship between individual chronotype (the result of the entrainment process) and longitude position in a time zone, using 12 years (2003–2014) of pooled diary data (n = 50,753) from the American Time Use Survey (ATUS). None of the studies addressing this question have been conducted in the US in a large-scale, nationally representative sample. The discrepancy between the natural light–dark cycle and self-selected light exposure has raised the question whether humans entrain to sun time (as most organisms do) vs. The availability of electrical light has altered modern light exposure, affecting the synchronization process (‘entrainment’) of the circadian clock to the natural light–dark cycle.
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