Seasonal synchronization of sleep timing in industrial and pre-industrial societies: the regulating role of Daylight Saving Time

Abstract

Did artificial light reshape human sleep/wake cycle? Most likely the answer is "yes". Did artificial light misalign the sleep/wake cycle in industrialized societies relative to the natural cycle of light and dark? For the average person -that is, obviating the tail of the distributionsthe answer is probably not.Sleep timing in extratropical, industrial (data from eight national time use surveys) societies and Subtropical, pre-industrial societies (data from nine locations coming from seven previous reports) with and without access to artificial light finds in winter a remarkable accommodation with sunrise time along a wide range of angular distance to the Equator (0 • to 55 • ).Within the two process model of sleep, results show sleep onset and sleep offset keep bound to each other by the homeostatic process, while the photoreceptive process aligns the phase of the sleep/wake cycle to winter sunrise. This results in a phase increasingly lagging with increasing latitude up to a delay of 120 min at 55 • latitude.In the summer season, the homeostatic process persists binding sleep onset to speep offset but Daylight Saving Time regulations in industrialized societies reduce the lag to 40 min at 55 • latitude. Sleep timing is then stationary with latitude.