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, or hypersomnia. For the sleep-deprived, microsleeping can occur at any time, typically without substantial warning.
Microsleeps become extremely dangerous when they occur in 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.