Hanyang Med Rev.  2013 Nov;33(4):190-196. 10.7599/hmr.2013.33.4.190.

The Physiology of Normal Sleep

• Koo DL
• Kim J

• KMID: 2284433
• DOI: http://doi.org/10.7599/hmr.2013.33.4.190

Abstract

Sleep is a highly organized and complicated state that is fundamental to life. We have an absolute need to sleep during about one-third of our lives. There are two types of sleep, non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. NREM sleep is divided into stages 1, 2, and 3 which is representing a degree of relative depth in sleep. Each sleep stage shows unique features including some variations in electroencephalographic waves, eye movements, and muscle tone. Although sleep pattern changes are associated with aging, how sleep physiology and sleep patterns change over an individual's life span is not well-defined. Circadian rhythms, which are the daily rhythms in physiology and behavior, regulate the sleep-wake cycle. Comprehensive understanding of normal sleep physiology should be very important to better understand not only the effects of sleep related diseases but also the impacts of pathological sleep on various diseases of other systemic organs. This review aims to enhance knowledge focused on normal sleep physiology and its regulation.

Keyword

Sleep; Physiology; Circadian Rhythm; Neurobiology

MeSH Terms

Aging
Circadian Rhythm
Eye Movements
Muscles
Neurobiology
Physiology*
Sleep Stages
Sleep, REM

Figure

• 

  Fig. 1 The hypnogram of a healthy adult represents a normal sleep architecture. There are 4 to 6 sleep cycles during the night. Slow wave sleep (stage N3) is longer early in the sleep period. Rapid eye movement sleep (stage R) increases in frequency and length later during the sleep period. Brief awakenings can be present normally during the sleep period.

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  Fig. 2 Typical polysomnographic findings during wake and sleep stages. A. Stage W: Alpha rhytms is the marker for wake stage. B. Stage N1 and N2 sleep: Slow eye movements, low amplitude and mixed frequency activity, vertex sharp waves indicates stage N1 sleep. K complex and sleep spindle are observed during stage N2 sleep. C. Stage N3 sleep: More than 20% of an epoch consists of slow wave activity during stage N3 sleep. D. Stage R sleep: Rapid eye movements and low chin electromyography (EMG) tone are characteristic features of stage R sleep.

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  Fig. 3 Two-process model of sleep regulation. Process S indicates the homeostatic built-up of sleep pressure and Process C represents the circadian rhythm.

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  Fig. 4 Sleep and age. WASO, wakefulness after sleep onset; REM, rapid eye movement. Ref. 19 with permission from The Korean Neurological Association.

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  Fig. 5 Sleep-wake regulation: 'flip-flop' switch model. I, inhibition; S, stabilization; VLPO, ventrolateral preoptic nucleus; TMN, tuberomammillary nucleus; LH, lateral hypothalamus; Raphe, raphe nuclei; LC, Locus coeruleus.

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