Most of the evidence for this pattern of segmented sleep is collected in historian Roger Ekirch’s At Day’s Close: Night in Times Past. After twenty years of research, Ekirch uncovered hundreds of references to segmented sleep in diaries, legal depositions, medical books, and literature, mostly in the British Isles but also beyond. Ekirch’s evidence suggests that for centuries, perhaps millennia, people went to bed a few hours after dusk, rose sometime after midnight for an hour or two of quiet activity, then went back to bed until early morning.
The notion that humans had two sleeps prior to industrialisation has been the subject of increasing media attention in the past year or so. The latest is this piece in The Guardian where the author began experimenting with segmented sleep and found it drastically improved his well-being. Many of the comments are skeptical as to how widespread segmented sleep was in reality but the evidence for it seems compelling (as documented in this Lapham’s Quarterly piece from which the above quote is taken).