Definition reprinted with permission from Culebras, Antonio, Clinical Handbook of Sleep Disorders, Butterworth-Heinemann
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The transition from awake to sleep, normally to non-REM stage 1 sleep, but in certain conditions, such as infancy and narcolepsy, into stage REM sleep. Most polysomnographers accept EEG slowing, reduction, and eventual disappearance of alpha activity, presence of EEG vertex sharp transients, and slow rolling eye movements (the components of non-REM stage 1) as sufficient for sleep onset; others require appearance of stage 2 patterns. See Latency; Sleep Stages.