A term introduced in or shortly before 1966 by the Russian-American neurophysiologist Wladimir Theodore Liberson (1904-1994) to denote a sleep state lasting for a short lapse of time, typically a few seconds to a minute. The definition and diagnostic criteria of the microsleep tend to differ somewhat across authors, and consensus on how to assess them is lacking. Some authors assess microsleeps on the basis of clinical signs such as a blank stare, drooping eyelids, prolonged eye closure, hypnagogic jerks, unintended paroxysmal vocalizations, snoring, and cataplexy, whereas others rely on the electroencephalogram (EEG) to assess the microsleep. On the EEG, microsleeps are characterized by a lowering of alpha activity (i.e. the activity characteristic of the waking state) and the appearance of theta activity (associated with stage N1 sleep). Microsleeps can occur in any one person, especially in a warm and poorly ventilated environment, during the performance of monotonous tasks, and during times of the day when the brain is 'programmed' to enter the sleep state, such as the pre-dawn and mid-afternoon hours. The incidence of microsleeps tends to be heightened in individuals suffering from conditions such as * sleep deprivation, physical or mental fatigue, the sleep apnea syndrome, and narcolepsy. During microsleeps the affected individual may enter the * dream state or the hypnagogic state virtually without a threshold, and without being aware of a discontinuity in the perceptual and cognitive state. This may entail the experiencing of dream images or * hypnagogic hallucinations, and lead up to life-threatening situations in individuals operating a machine or vehicle, for example. It is not unthinkable that the * daymare (i.e. the daytime equivalent of the classical * nightmare) and the microsleep have certain neurophysiological correlates in common.
   Harrison, Y., Horne, J.A. (1996). Occurrence of 'microsleeps' during daytime sleep onset in normal subjects. Electroencephalography and Clinical Neurophysiology, 98, 411-416.
   Liberson, W.T., Liberson, C.T. (1966). EEG recordings, reaction times, eye movements, respiration and mental content during drowsiness. Proceedings of the Society for Biological Psychiatry, 19, 295-302.

Dictionary of Hallucinations. . 2010.

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