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1.
The kind of involuntary sleep you experience when you're really tired and trying to stay awake in a monotonous situation, like a class lecture. One minute you're propping your head in your hand, trying to pay attention, when all of a sudden you're out -- at least until your elbow slips sideways or your head falls forward, jarring you back to attention. Caused by a consistent lack of sleep over several days. Often the cause of car accidents, when the driver nods off during long hauls.
Man, I had a really bad microsleep during trig yesterday. I couldn't stay awake for more than a minute or two.
by W@tcher January 09, 2006
 
2.
Usually results from sleep deprivation. Microsleep may be linked to narcolepsy and other sleep disorders such as severe insomnia. The person may believe they are awake but are actually in the REM cycle of the sleep process. It happens so quick that the person may not even be aware that they are sleeping. Episodes can last from 30 seconds to a few minutes. Usually, the person enters the dream state and may slip in and out of the REM cycle periodically.
I just fell asleep for 30 seconds, I just had a microsleep episode.
by NND9987 February 25, 2010