For many centuries, and perhaps back to Homer, Western society slept in two shifts. People went to sleep, got up in the middle of the night for an hour or so, and then went to sleep again. Thus night — divided into a “first sleep” and “second sleep” — also included a curious intermission. “There was an extraordinary level of activity,” Ekirch told me. People got up and tended to their animals or did housekeeping. Others had sex or just lay in bed thinking, smoking a pipe, or gossiping with bedfellows. Benjamin Franklin took “cold-air baths,” reading naked in a chair.
Via Improbable Research.
According to Wikipedia, this phenomenon is called segmented sleep:
Segmented sleep, divided sleep, bimodal sleep pattern and interrupted sleep are modern Western terms for a polyphasic sleep pattern found in medieval and early modern Europe and many non-industrialised societies today, where the night's sleep is divided by one or more periods of wakefulness. This is particularly common in the winter.
The human circadian rhythm controls a sleep-wake cycle of wakefulness during the day and sleep at night. Superimposed on this basic rhythm is a secondary one of light sleep in the early afternoon (see siesta) and quiet wakefulness in the early morning.
The two periods of night sleep were called first sleep (occasionally dead sleep) and second sleep (or morning sleep) in medieval England. First and second sleep are also the terms in the Romance languages as well as the Tiv of Nigeria: In French, the common term was premier sommeil or premier somme; in Italian, primo sonno or primo sono; in Latin, primo somno or comcubia nocte. There is no common word in English for the period of wakefulness between, apart from paraphrases such as first waking or when one wakes from his first sleep and the generic watch (in its old meaning of being awake). In French an equivalent generic term is dorveille ("twixt sleep and wake").