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A microsleep is an episode of sleep which may last for a fraction of a second or up to thirty seconds. Often, it is the result of sleep deprivation, mental fatigue, depression, sleep apnea, hypoxia, narcolepsy, or hypersomnia. Microsleeping can occur at any time, typically without significant warning.

Microsleeps (or microsleep episodes) become extremely dangerous when occurring during situations which demand constant alertness, such as driving a motor vehicle or working with heavy machinery. People who experience microsleeps usually remain unaware of them, instead believing themselves to have been awake the whole time, or to have temporarily lost focus.

When experiencing microsleeps while driving an automobile, from the perspective of the driver, he or she drives a car, and then suddenly realizes that several seconds have passed by unnoticed. It is not obvious to the driver that he or she was asleep during those missing seconds, although this is in fact what happened. The sleeping driver is at very high risk for having an accident during a microsleep episode.

Many accidents and catastrophes have resulted from microsleep episodes in these circumstances. For example, a microsleep episode is claimed to have been one factor contributing to the Waterfall train disaster in 2003; the driver had a heart attack and the guard who should have reacted to the train's increasing speed is said by his defender to have microslept, thus causing him to be held unaccountable.

There is little agreement on how best to identify microsleep episodes. Some experts define microsleep according to behavioral criteria (head nods, drooping eyelids, etc.), while others rely on EEG markers. One study at the University of Iowa defined EEG-monitored microsleeps in driving simulation as "a 3–14 second episode during which 4–7 Hz (theta) activity replaced the waking 8–13 Hz (alpha) background rhythm."