In cats, the sleep cycle lasts about 30 minutes, though it is about 12 minutes in rats and up to 120 minutes in elephants. In infants, the sleep cycle lasts about 50–60 minutes average length increases as the human grows into adulthood. The captive Asian elephant (pictured) is thought to have a sleep cycle of 72 minutes. Other sources give 90–110 minutes or 80–120 minutes. The figure of 90 minutes for the average length of a sleep cycle was popularized by Nathaniel Kleitman around 1963. The brain is awake and body paralyzed." This unique stage usually occurs when the person dreams. They describe REM sleep as "A unique state, in which dreams usually occur. The National Sleep Foundation discusses the different stages of NREM sleep and their importance. Every part of the body is now relaxed, breathing, blood pressure and body temperature are reduced. N3 or even N4 are the most difficult stages to be awakened. The heart rate and body temperature also goes down. Brain wave frequency and muscle tonus is decreased. N2 is when the person experiences a light sleep. Brain waves and muscle activity start to decrease at this stage. N1 (NREM stage 1) is when the person is drowsy or awake to falling asleep. The standard figure given for the average length of the sleep cycle in an adult man is 90 minutes. Length Schematic illustration of a normal sleep cycle 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). The reciprocal interaction model proposed in the 1970s suggested a cyclic give-and-take between these two systems. Monoamines are active during NREMS, but not REMS, whereas acetylcholine is more active during REMS. 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. In humans, the transition between non-REM and REM is abrupt in other animals, it is less so. Alternation between the stages therefore maintains body temperature within an acceptable range. Thus, during REM sleep, body temperature tends to drift away from its mean level, and during non-REM sleep, to return to normal. Homeostatic functions, especially thermoregulation, occur normally during non-REM sleep, but not during REM sleep. 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). ĮOG (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. 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. In order to determine in which stage of sleep the asleep subject is, electroencephalography is combined with other devices used for this differentiation. Heart rate variability, well known to increase during REM, predictably also correlates inversely with delta-wave oscillations over the ~90-minute cycle. Secretions of various hormones, including renin, growth hormone, and prolactin, correlate positively with delta-wave activity, while secretion of thyroid-stimulating hormone correlates inversely. Delta wave activity, correlating with slow-wave (deep) sleep, in particular shows regular oscillations throughout a good night's sleep. Characteristics Įlectroencephalography shows the timing of sleep cycles by virtue of the marked distinction in brainwaves manifested during REM and non-REM sleep. These fluctuations may persist during wakefulness as rest-activity cycles but are less easily discerned. Within the sleep of adults and infants there are cyclic fluctuations between quiet and active sleep. In humans, this cycle takes 70 to 110 minutes (90 ± 20 minutes). 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. The sleep cycle is an oscillation between the slow-wave and REM (paradoxical) phases of sleep. Sample hypnogram showing one sleep cycle (the first of the night) from NREM through REM
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