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. Along with a nap (siesta) in the day, it has been argued that this is the natural pattern of human sleep. A case has been made that maintaining such a sleep pattern may be important in regulating stress.
Historian A. Roger Ekirch argues that before the Industrial Revolution, segmented sleep was the dominant form of human slumber in Western civilization. He draws evidence from documents from the ancient, medieval, and modern world, which he discovered over the course of fifteen years of research. Other historians, such as Craig Koslofsky, have endorsed Ekirch’s discovery and analysis.
According to Ekirch’s argument, typically individuals slept in two distinct 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, awoke later to have sex. People also used this time to pray and reflect, 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, or engaged in petty crime. The human circadian rhythm regulates the human sleep-wake cycle of wakefulness during the day and sleep at night. Ekirch suggests that it is due to the modern use of electric lighting that most modern humans do not practice segmented sleep, which is a concern for some scientists. Superimposed on this basic rhythm is a secondary one of light sleep in the early afternoon and quiet wakefulness in the early morning.
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 hypnagogic 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. If Ekirch’s theory is correct, their concerns might best be addressed by assurance that their sleep conforms to historically natural sleep patterns.
The two periods of night sleep in Ekirch’s theory were called ‘first sleep’ (occasionally ‘dead sleep’) and ‘second sleep’ (or ‘morning sleep’) in medieval England. Ekirch finds that first and second sleep were 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.’ He found 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’). Ekirch suggests that, because members of modern industrialized 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.’
The Daily Omnivore 
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