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.