Normal sleep is divided into non–rapid eye movement (NREM) and REM sleep. NREM sleep is further divided into progressively deeper stages of sleep: stage N1, stage N2, and stage N3 (deep or delta-wave sleep). Historically, sleep was thought to be a passive state that was initiated through withdrawal of sensory input. Currently, withdrawal of sensory awareness is believed to be a factor in sleep, but an active initiation mechanism that facilitates brain withdrawal is also recognized. Both homeostatic factors (factor S) and circadian factors (factor C) interact to determine the timing and quality of sleep.
Stage N1 is considered a transition between wake and sleep. It occurs upon falling asleep and during brief arousal periods within sleep and usually accounts for 2%-5% of total sleep time. Stage N2 occurs throughout the sleep period and represents 45%-55% of total sleep time. Stage N3 (delta or slow wave sleep) occurs mostly in the first third of the night and constitutes 5%-15% of total sleep time. REM represents 20%-25% of total sleep time and occurs in 4-5 episodes throughout the night.
Typically, N3 sleep is present more in the first third of the night, whereas REM sleep predominates in the last third of the night. This can be helpful clinically as NREM parasomnias such as sleep walking typically occur in the first third of the night with the presence of N3 sleep. This contrasts with REM sleep behavior disorder (RBD), which typically occurs in the last half of the night.
Sleep deprivation is a relative concept. Small amounts of sleep loss (eg, 1 hour per night over many nights) have subtle cognitive costs, which appear to go unrecognized by the individual experiencing the sleep loss. More severe restriction of sleep for a week leads to profound cognitive deficits similar to those seen in some stroke patients, which also appear to go unrecognized by the individual. The lack of recognition of the effects of sleep deprivation appears to be a constant feature, one which, it is hoped, will be overcome by further research and education. Short-term sleep deprivation has been implicated in contributing to obesity as well as glycemic dysregulation, contributing to poor control of type 2 diabetes.
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