Segmented sleep, divided sleep, bimodal sleep pattern and interrupted sleep are modern Western terms for a polyphasic or biphasic 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; 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").
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 reflect, and to 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 scholars and poets to write uninterrupted.
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.
Because members of modern industrialised 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.
The modern assumption that consolidated sleep with no awakenings is the normal and correct way for human adults to sleep leads many 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.