At Day's Close: Night in Times Past, which is about night in preindustrial times, A. Roger Ekirch, professor of history at Virginia Polytechnic Institute in Roanoke, uncovered the fact that in preindustrial times before artificial illumination was widely used, persons typically slept in 2 shifts.1 They called the shifts "first sleep" and "second sleep." In those times, sleep was more closely associated with sunset and sunrise than it is now. Within an hour or so after sunset, persons retired to bed, slept for about 4 hours and then woke up. They remained awake for a few hours and then returned to sleep at about 2 am for another 4 hours or so...

Although diaries, court documents, and literature of the time indicate that this sleep pattern was widely acknowledged, this bit of history had been lost until the debut of Ekirch's work. The pattern of sleep he describes as the norm in days past is no longer the norm in developed countries where artificial light extends the day. Anthropologists, however, have observed a similar pattern of segmented sleep among some contemporary African tribes,1 such as the Tiv of central Nigeria, who even refer to their customary sleep patterns as first sleep and second sleep, just as the early Europeans did.