The dominant pattern of sleep, arguably since time immemorial, was biphasic," Roger Ekirch, a sleep historian at Virginia Tech University and author of "At Day's Close: Night in Times Past" (Norton 2005).
"Humans slept in two four-hour blocks, which were separated by a period of wakefulness in the middle of the night lasting an hour or more. During this time some might stay in bed, pray, think about their dreams, or talk with their spouses. Others might get up and do tasks or even visit neighbors before going back to sleep."
References to "first sleep" or "deep sleep" and "second sleep" or "morning sleep" abound in legal depositions, literature and other archival documents from pre-Industrial European times. Gradually, though, during the 19th century, "language changed and references to segmented sleep fell away," said Ekirch. "Now people call it insomnia."