Chronotypes in the US: Influence of longitude position in a time zone.

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. 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. social time. None of the studies addressing this question have been conducted in the US in a large-scale, nationally representative sample. 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). Chronotype was estimated based on mid-sleep time on weekends (MSF), a proxy that was previously shown to replicate known age and sex differences in chronotype in the ATUS. Longitude position was derived from state-level information (e.g., average state border outline). Regression results showed a progressive delay in MSF 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 < .01). 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. 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.