Micro-sleeps 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. These are potentially among the most dangerous consequences of insomnia.
Microsleep episodes last from a few seconds to two 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.
Microsleep periods become more prevalent with cumulative sleep debt. In other words, the more sleep deprived a person is, the greater the chance a microsleep episode will occur.
It is not clear what happens in the brain during microsleep. It appears that some part of the brain effectively falls asleep while the rest is awake. This could account for selective loss of awareness without the person feeling he or she has been asleep.
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 of insomnia, is not the same as micro-sleep, although micro-sleep episodes often occur during periods of Excessive Daytime Sleepiness. There is a reduction in vigilance – that elusive characteristic of wakefulness.
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 microsleep. The EEG readings are of limited use because the changes during microsleep are subtle. German scientists have attempted to use detection of “alpha events” to identify microsleep. The EEG is more useful at a macro-level while microsleep is characterized by sections of the brain falling asleep while others stay awake.
There is some evidence that parts of the brain are sleep briefly during the day when a sleep-deprived person is awake. There is lower activity shown in the EEG readings. Animal studies show that in sleep-deprived individuals slower brain wave activity typical of sleep (delta, < 4 Hz; or theta, 4–7 Hz) leaks into periods during which the animal may be moving around with eyes open,
But, scientists still don’t understand what is going on. Is it selective regional sleep where some parts of the brain sleep? If so, this defies our commonly understood definition of sleep as a state of the brain as a whole. Is it a checker-board pattern where individual neurons are awake (depolarized) and others are asleep (oscillate between up and downstates)? Is there some third state at a neuron level that is neither sleeping nor waking?