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Segmented sleep or divided sleep are modern Western terms for a sleep pattern found in medieval Europe and many modern non-industrial societies, where the night's sleep is evenly divided by a few hours of wakefulness.

The human Circadian rhythm controls a sleep-wake cycle of wakefulness during the day and sleep at night. Superposed on this basic rhythm is a secondary one of light sleep in the early afternoon (see nap and 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. 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 sleepe and wake").

This period of wakefulness was often only semi-conscious, as the French term implies. It was highly valued in medieval Europe as a time of quiet and relaxation. Peasant couples were often too tired after a long day's work to do much more than eat and go to sleep, but they would wake later on to talk and make love. People would also use this time to pray and interpret dreams, which were more vivid at that hour than upon waking in the morning, and even to visit. This was also a favorite time for authors and poets to write uninterrupted.

There is evidence from sleep research that this period of nighttime wakefulness, combined with a midday nap, result in greater alertness than a single sleep-wake cycle. The brain exhibits high levels of the pituitary hormone prolactin during the period of nighttime wakefulness, which may contribute to the feeling of peace that many people associate with it. It is in many ways similar to the hypnogogic and hypnopompic states which occur just before falling asleep and upon waking, repectively.

Because members of modern industrial societies, with late hours facilitated by electric lighting, no longer have this sleep pattern, they may misinterpret and mistranslate references to it in literature. Common interpretations of the term 'first sleep' are 'beauty sleep' and 'early slumber'. A reference to first sleep in the Odyssey was translated as such in the 17th century, but universally mistranslated in the 20th.

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