Polyphasic sleep, a term coined by early 20th century psychologist J.S. Szymanski, refers to the practice of sleeping multiple times in a 24-hour period—usually, more than two, in contrast to biphasic sleep —and does not imply any particular schedule. See also Segmented sleep and Sleep (Optimal amount). The term polyphasic sleep is also used by an online community which experiments with ultra-short napping to achieve more time awake each day.

An example of polyphasic sleep is found in patients with irregular sleep-wake pattern, a circadian rhythm sleep disorder which usually is caused by head injury or dementia. Much more common examples are the sleep of human infants and of many animals. Elderly humans often have disturbed sleep, including polyphasic sleep.

In their 2006 paper “The Nature of Spontaneous Sleep Across Adulthood”, Campbell and Murphy studied sleep timing and quality in young, middle-aged and older adults. They found that, in freerunning conditions, the average duration of major nighttime sleep was significantly longer in young adults than in the other groups. The paper states further:

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