Segmented sleep, also known as divided sleep, bimodal sleep pattern, or interrupted sleep, is a polyphasic or biphasic sleep pattern where two or more periods of sleep are punctuated by a period of wakefulness.
In Western civilization before the Industrial Revolution, segmented sleep was the dominant form of human slumber since time immemorial, according to A. Roger Ekirch, a historian at Virginia Tech. Over the course of nearly two decades of research, Ekirch discovered extensive evidence of this sleep pattern in a wide variety of documents, dating from the ancient world until the nineteenth century. Typically, individuals slept in two discrete phases, bridged by an intervening period of wakefulness of up to an hour or more. Peasant couples, who were often too tired after field labor to do much more than eat and go to sleep, awakened later to have sex.[1] People also used this time to pray and reflect,[2] and to interpret dreams, which were more vivid at that hour than upon waking in the morning. This was also a favorite time for scholars and poets to write uninterrupted, whereas still others visited neighbors and engaged in petty crime.[3]
The human circadian rhythm regulates the human sleep-wake cycle of wakefulness during the day and sleep at night. Due to the modern use of electric lighting, most modern humans do not practice segmented sleep, which is a concern for some scientists.[4] 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.
There is evidence from sleep research that this period of nighttime wakefulness, combined with a midday nap, results in greater alertness than a monophasic 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, respectively.
The modern assumption that consolidated sleep with no awakenings is the normal and correct way for human adults to sleep may lead many people to approach their doctors with complaints of maintenance insomnia or other sleep disorders. Their concerns might best be addressed by assurance that their sleep conforms to historically natural sleep patterns.[5]
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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; in Latin, primo somno or comcubia nocte.[6] 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").
Because members of modern industrialised societies, with late hours facilitated by electric lighting, mostly do not practice segmented sleep, 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.[7]