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The sleep cycle is an oscillation between the slow-wave sleep and REM (paradoxical) phases of sleep. It is sometimes called the ultradian sleep cycle, sleep–dream cycle, or REM-NREM cycle, to distinguish it from the circadian rhythm alternation between sleep and wakefulness. In humans, this cycle takes 70 to 110 minutes (90 ± 20 minutes). Within the sleep of adults and infants there are cyclic fluctuations between quiet and active sleep. These fluctuations may persist during wakefulness as rest-activity cycles but are less easily discerned.
In order to determine in which stage of sleep the asleep subject is, electroencephalography is combined with other devices used for this differentiation. EMG (electromyography) is a crucial method to distinguish between sleep phases: for example, a decrease of muscle tone is in general a characteristic of the transition from wake to sleep,Kleitman, N. (1963). Sleep and Wakefulness Chicago, Univ. Chicago Jfress and during REM sleep, there is a state of muscle atonia (paralysis), resulting in an absence of signals in the EMG.
EOG (electrooculography), the measure of the eyes’ movement, is the third method used in the sleep architecture measurement;Berry, R. B., & Wagner, M. H. (2014). Sleep Medicine Pearls E-Book. Elsevier Health Sciences. for example, REM sleep, as the name indicates, is characterized by a rapid eye movement pattern, visible thanks to the EOG.ber C., Ancoli-Israel S., Chesson A., and Quan SF. in The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specifications, 1st. Ed.: Westchester, Illinois: American Academy of Sleep Medicine; 2007.
Moreover, methods based on cardiorespiratory parameters are also effective in the analysis of sleep architecture—if they are associated with the other aforementioned measurements (such as electroencephalography, electrooculography and the electromyography).Tataraidze, A., Korostovtseva, L., Anishchenko, L., Bochkarev, M., & Sviryaev, Y. (2016,). Sleep architecture measurement based on cardiorespiratory parameters.
Homeostasis functions, especially thermoregulation, occur normally during non-REM sleep, but not during REM sleep. Thus, during REM sleep, body temperature tends to drift away from its mean level, and during non-REM sleep, to return to normal. Alternation between the stages therefore maintains body temperature within an acceptable range.Pier Luigi Parmeggiani, "Modulation of body core temperature in NREM sleep and REM sleep"; in Mallick et al. (2011).
In humans, the transition between non-REM and REM is abrupt; in other animals, it is less so.
Researchers have proposed different models to elucidate the undoubtedly complex rhythm of electrochemical processes that result in the regular alternation of REM and NREM sleep. are active during NREMS, but not REMS, whereas acetylcholine is more active during REMS. The reciprocal interaction model proposed in the 1970s suggested a cyclic give-and-take between these two systems. More recent theories such as the "flip-flop" model, proposed in the 2000s, include the regulatory role of an inhibitory neurotransmitter gamma-aminobutyric acid (GABA).James T. McKenna, Lichao Chen, & Robert McCarley, "Neuronal models of REM-sleep control: evolving concepts"; in Mallick et al. (2011).
In , the sleep cycle lasts about 50–60 minutes; average length increases as the human grows into adulthood. In , the sleep cycle lasts about 30 minutes, though it is about 12 minutes in and up to 120 minutes in (In this regard, the ontogeny of the sleep cycle appears proportionate with metabolism processes, which vary in proportion with organism size. However, shorter sleep cycles detected in some elephants complicate this theory).
The cycle can be defined as lasting from the end of one REM period to the end of the next, or from the beginning of REM, or from the beginning of non-REM stage 2 (the decision of how to mark the periods makes a difference for research purposes, because of the unavoidable inclusion or exclusion of the night's first NREM or its final REM phase if directly preceding Wakefulness).
A 7–8-hour sleep probably includes five cycles, the middle two of which tend to be longer than the first and the fourth. REM takes up more of the cycle as the night goes on.Daniel Aeschbach, "REM-sleep regulation: circadian, homeostatic, and non-REM sleep-dependent determinants"; in Mallick et al. (2011).
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