Sleep cycle

[3] Heart rate variability, well known to increase during REM, predictably also correlates inversely with delta-wave oscillations over the ~90-minute cycle.

[4] In order to determine in which stage of sleep the asleep subject is, electroencephalography is combined with other devices used for this differentiation.

[8] 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).

[11] 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.

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).

[4] In infants, the sleep cycle lasts about 50–60 minutes; average length increases as the human grows into adulthood.

[11][16] Unprovoked awakening occurs most commonly during or after a period of REM sleep, as body temperature is rising.

[17] 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.

[13] According to this hypothesis, during the period of this cycle corresponding with REM, people tend to daydream more and show less muscle tone.

[14][19] 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.