Segmented Sleep

Segmented sleep is a pattern in which night sleep divides into two bouts — a first sleep and a second sleep — separated by an hour or more of quiet wakefulness. The historian A. Roger Ekirch assembled the case in At Day's Close (2005), citing several hundred references across court depositions, diaries, medical texts and literature in which the interval is treated as unremarkable: people prayed, talked, tended fires, or lay still until the second sleep took them.

Laboratory work gave the idea a physiological anchor. Thomas Wehr's early-1990s experiments placed participants in fourteen hours of darkness nightly; after some weeks their sleep tended to split into two bouts around a wakeful interval marked by elevated prolactin. That is a real result about long nights, and it makes the historical pattern credible.

The dispute is over universality. Ekirch's evidence is documentary and drawn largely from temperate pre-industrial Europe, where winter nights are long; the sources are also indirect, which raises the usual questions of Provenance. Field studies of societies without electric light — including Tanzanian, Namibian and Bolivian groups — found consolidated single sleep bouts, arguing that segmentation answers long dark nights rather than expressing a human default. The strong reading, that modern consolidated sleep is unnatural, outruns its evidence and is better held as Plausible Mechanism than fact; the narrow reading survives contact with Falsifiability.

What electric light changed is not in doubt. Evening illumination suppresses melatonin and delays circadian phase, compressing the biological night, an effect compounded by clock conventions covered in Time Zone. Nocturnal waking is common and usually benign; sedative-hypnotics used against it carry the issues in Physical Dependence.