<unk>  sleep (or<unk> , bifurcated, or bimodal sleep) is the practice of sleeping during two periods over the course of 24 hours, while<unk>  sleep refers to sleeping multiple times—usually more than two. Each of these is in contrast to monophasic sleep, which is one period of sleep within 24 hours. Segmented sleep and divided sleep may refer to<unk>  or<unk>  sleep, but may also refer to interrupted sleep, where the sleep has one or several shorter periods of wakefulness, as was the norm for night sleep in pre-industrial societies. A common form of<unk>  or<unk>  sleep includes a nap, which is a short period of sleep, typically taken between the hours of 9 am and 9 pm as an adjunct to the usual nocturnal sleep period. Napping behaviour during daytime hours is the simplest form of<unk>  sleep, especially when the nap(s) are taken on a daily basis. The term<unk>  sleep was first used in the early 20th century by psychologist J. S. Szymanski, who observed daily fluctuations in activity patterns (see Stampi 1992). It does not imply any particular sleep schedule. The circadian rhythm disorder known as irregular sleep-wake syndrome is an example of<unk>  sleep in humans.<unk>  sleep is common in many animals, and is believed to be the ancestral sleep state for mammals, although simians are monophasic. The term<unk>  sleep is also used by an online community that experiments with alternative sleeping schedules to achieve more wake time and/or better sleep each day.
Biphasic and polyphasic sleep
Non-rapid eye movement sleep
Rapid eye movement sleep
Free-running sleep
Slow-wave sleep
Postictal state
Twilight sleep
Intermittent rhythmic delta activity