a moment of sleep followed by disorientation, experienced especially by persons with narcolepsy or those affected by sleep deprivation.
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British Dictionary definitions for microsleep
a period of sleep which is so momentary as to be imperceptible
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012