Segmented sleep

Segmented sleep, also known as divided sleep, bimodal sleep pattern, bifurcated sleep, or interrupted sleep, is a polyphasic or biphasic sleep pattern where two or more periods of sleep are punctuated by periods 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 has argued 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. Other historians, such as Craig Koslofsky, have endorsed Ekirch's analysis.

Segmented sleep as a historical norm
According to Ekirch's argument, typically individuals slept in two distinct phases, bridged by an intervening period of wakefulness of approximately one hour. 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, had sex, 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 popular assumption, that consolidated sleep with no awakenings is the normal and correct way for human adults to sleep, may lead people to consult their doctors fearing they have maintenance insomnia or other sleep disorders. If Ekirch's hypothesis is correct, their concerns might best be addressed by reassurance 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; in Italian, primo sonno; in Latin, primo somno or comcubia nocte. 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 (a portmanteau of the French words dormir (“to sleep”) and veiller (“to be awake; to be alert”)).

Ekirch suggests that, 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 seventeenth century, but, if Ekirch's hypothesis is correct, was universally mistranslated in the twentieth.

Wehr's study
In one experiment Thomas Wehr had eight healthy men be confined to a room for fourteen hours of darkness every day for a month. At first the participants slept for about eleven hours, presumably making up for their sleep debt. After this the subjects began to sleep much as people in pre-Industrial times had. They would sleep for about four hours, wake up for two to three hours, then go back to bed for another four hours. They also took about two hours to fall asleep.

Physiology
In Dreamland: Adventures in the Strange Science of Sleep, David Randall discusses how during the nocturnal waking period of a segmented sleep the hormone prolactin is released, along with other physiological changes. Prolactin is released with the relaxation that accompanies orgasm.