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 distributions--- the answer is probably not. Sleep timing in industrial (data from eight national time use surveys) and pre-industrial, hunter-gatherer/horticulturalist societies (seven data from three previous reports) with and without access to artificial light across a wide range of angular distance to the Equator (0° to 55°) finds a remarkable accommodation in trends dominated by the light/dark cycle. Daylight saving time (DST) in modern societies helps swinging synchronization through seasons. During standard time, winter sunrise synchronizes sleep timing in the observed range of angular distance to the Equator. That means sleep timing delays with increasing distance to the Equator. DST mitigates this delay and makes summer sleep timing is less influenced by distance to the Equator.
Footnotes
Sleep timing in NTUS/Industrial societies has been recomputed. Earlier versions of the manuscript incorrectly averaged sleep/wake daily rhythms as week-end diaries were largely overpopulated. Figure 1, and tabular material have been updated accordingly. Title, abstract, discussion and conclusion have been rephrased.
