Glossary of Sleep-related terms

Sleep Onset

The time from when a person attempts to fall asleep until the onset of sleep. This sleep onset normally leads to NREM stage 1 sleep, but in certain conditions, such as infancy and narcolepsy, may lead to REM stage sleep. In polysomnography,1 sleep onset is the point at which the first polygraphically recorded epoch of sleep (i.e., stage 1, 2, 3, 4, or REM) was recorded within a sleep episode. Any modification of this should be explicitly defined (e.g., 'sleep onset (s-2)' to indicate first epoch of stage 2 sleep).3