Microsleeps are brief, unintended episodes of loss of attention associated with events such as blank stare, head snapping, and prolonged eye closure which may occur when a person is fatigued but trying to stay awake to perform a monotonous task like driving a car or watching a computer screen.
Microsleep episodes last from a few seconds to several minutes, and often the person is not aware that a microsleep has occurred. In fact, microsleeps often occur when a person's eyes are open. While in a microsleep, a person fails to respond to outside information. A person will not see a red signal light or notice that the road has taken a curve, which is why this phenomenon is of particular interest to people who study drowsy driving. During a microsleep, a pilot might not be aware of flashing alarm lights in the cockpit.
Microsleeps are most likely to occur at certain times of the day, such as pre-dawn hours and mid-afternoon hours when the body is "programmed" to sleep. Microsleeps increase with cumulative sleep debt. In other words, the more sleep deprived a person is, the greater the chance a microsleep episode will occur.
People with sleep disorders often experience microsleeps, but pretty much everyone can have them, particularly if they are tired. It should be noted that “Excessive Daytime Sleepiness”, a well recognized symptom, is not the same as microsleep, although microsleep episodes often occur during periods of Excessive Daytime Sleepiness.
Researchers have tried to quantify microsleep times and episodes in an attempt to develop a diagnostic tool like the commonly used multiple sleep latency test. However, this has proven difficult and at this time there is no agreed-upon clinical tool for assessing micosleep.