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The sleep cycle is an oscillation between the slow-wave and
REM Rem or REM may refer to: Music * R.E.M., an American rock band * ''R.E.M.'' (EP), by Green * "R.E.M." (song), by Ariana Grande Organizations * La République En Marche!, a French centrist political party * Reichserziehungsministerium, in Nazi G ...
(paradoxical) phases of
sleep Sleep is a sedentary state of mind and body. It is characterized by altered consciousness, relatively inhibited Perception, sensory activity, reduced muscle activity and reduced interactions with surroundings. It is distinguished from wakefuln ...
. It is sometimes called the ultradian sleep cycle, sleep–dream cycle, or REM-NREM cycle, to distinguish it from the circadian alternation between sleep and wakefulness. In humans, this cycle takes 70 to 110 minutes (90 ± 20 minutes).


Characteristics

Electroencephalography Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocor ...
shows the timing of sleep cycles by virtue of the marked distinction in brainwaves manifested during REM and non-REM sleep.
Delta wave Delta waves are high amplitude neural oscillations with a frequency between 0.5 and 4 hertz. Delta waves, like other brain waves, can be recorded with electroencephalography (EEG) and are usually associated with the deep stage 3 of NREM slee ...
activity, correlating with slow-wave (deep) sleep, in particular shows regular oscillations throughout a good night's sleep. Secretions of various
hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...
s, including renin,
growth hormone Growth hormone (GH) or somatotropin, also known as human growth hormone (hGH or HGH) in its human form, is a peptide hormone that stimulates growth, cell reproduction, and cell regeneration in humans and other animals. It is thus important in ...
, and prolactin, correlate positively with delta-wave activity, while secretion of
thyroid-stimulating hormone Thyroid-stimulating hormone (also known as thyrotropin, thyrotropic hormone, or abbreviated TSH) is a pituitary hormone that stimulates the thyroid gland to produce thyroxine (T4), and then triiodothyronine (T3) which stimulates the metabolis ...
correlates inversely. Heart rate variability, well known to increase during REM, predictably also correlates inversely with delta-wave oscillations over the ~90-minute cycle. 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, 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; for example, REM sleep, as the name indicates, is characterized by a rapid eye movement pattern, visible thanks to the EOG. 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). Homeostatic functions, especially
thermoregulation Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
, 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. 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. Monoamines are active during NREMS, but not REMS, whereas
acetylcholine Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...
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).


Length

The standard figure given for the average length of the sleep cycle in an adult man is 90 minutes. N1 (NREM stage 1) is when the person is drowsy or awake to falling asleep. Brain waves and muscle activity start to decrease at this stage. N2 is when the person experiences a light sleep. Eye movement has stopped by this time. Brain wave frequency and muscle tonus is decreased. The heart rate and body temperature also goes down. N3 or even N4 are the most difficult stages to be awakened. Every part of the body is now relaxed, breathing, blood pressure and body temperature are reduced. The National Sleep Foundation discusses the different stages of NREM sleep and their importance. They describe REM sleep as "A unique state, in which dreams usually occur. The brain is awake and body paralyzed." This unique stage usually occurs when the person dreams. The figure of 90 minutes for the average length of a sleep cycle was popularized by Nathaniel Kleitman around 1963. Other sources give 90–110 minutes or 80–120 minutes. In
infant An infant or baby is the very young offspring of human beings. ''Infant'' (from the Latin word ''infans'', meaning 'unable to speak' or 'speechless') is a formal or specialised synonym for the common term ''baby''. The terms may also be used t ...
s, the sleep cycle lasts about 50–60 minutes; average length increases as the human grows into adulthood. In
cat The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of ...
s, the sleep cycle lasts about 30 minutes, though it is about 12 minutes in
rat Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include ''Neotoma'' ( pack rats), ''Bandicota'' (bandicoot ...
s and up to 120 minutes in
elephant Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantidae ...
s. (In this regard, the
ontogeny Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the stu ...
of the sleep cycle appears proportionate with
metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cel ...
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 awakening.) 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.


Awakening

Unprovoked awakening occurs most commonly during or after a period of REM sleep, as
body temperature Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperatur ...
is rising.


Continuation during wakefulness

Ernest Hartmann discovered in 1968 that humans seem to continue a roughly 90-minute ultradian rhythm throughout a 24-hour day, whether they are asleep or awake. According to this hypothesis, during the period of this cycle corresponding with REM, people tend to daydream more and show less muscle tone. Kleitman and others following have referred to this rhythm as the
basic rest–activity cycle The basic rest–activity cycle (BRAC) is a physiological arousal mechanism in humans proposed by Nathaniel Kleitman, hypothesized to occur during both sleep and wakefulness. Empirically, it is an ultradian rhythm of approximately 90 minutes (80� ...
, of which the "sleep cycle" would be a manifestation. A difficulty for this theory is the fact that a long non-REM phase almost always precedes REM, regardless of when in the cycle a person falls asleep.


Alteration

The sleep cycle has proven resistant to systematic alteration by
drug A drug is any chemical substance that causes a change in an organism's physiology or psychology when consumed. Drugs are typically distinguished from food and substances that provide nutritional support. Consumption of drugs can be via inhal ...
s. Although some drugs shorten REM periods, they do not abolish the underlying rhythm. Deliberate REM deprivation shortens the cycle temporarily, as the brain moves into REM sleep more readily (the "REM rebound") in an apparent correction for the deprivation. There are various methods to control the alterations of sleep cycles: * Switching off all artificial lights: Since the natural production of melatonin can be suppressed by bright light, exposing to light—even after sunset— may prevent the body from feeling sleepy (and hence entering the sleep phase). * Meditation and relaxation techniques: * Staying away from caffeine before bedtime: This ensures that the body is not under the stimulant effects of caffeine while trying to sleep.


See also

* Biphasic and polyphasic sleep *
Circadian rhythm A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., endogenous) and responds to ...


References


Bibliography

* Mallick, B. N.; S. R. Pandi-Perumal; Robert W. McCarley; and Adrian R. Morrison (2011). ''Rapid Eye Movement Sleep: Regulation and Function''. Cambridge University Press. {{ISBN, 978-0-521-11680-0 *Nir, and Tononi, "Dreaming and the Brain: from Phenomenology to Neurophsiology." ''Trends in Cognitive Sciences'', vol. 14, no. 2, 2010, pp. 88–100. *Varela, F., Engel, J., Wallace, B., & Thupten Jinpa. (1997). ''Sleeping, dreaming, and dying: An exploration of consciousness with the Dalai Lama''. Sleep physiology Chronobiology