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Initially, it was thought that sleep was simply a mechanism for the body to "take a break" and reduce wear. Later observations of the low metabolic rates in the brain during sleep seemed to indicate some metabolic functions of sleep. [109] This theory is not fully adequate as sleep only decreases metabolism by about 5–10%.
The electrical activity seen on an EEG represents brain waves. The amplitude of EEG waves at a particular frequency corresponds to various points in the sleep-wake cycle, such as being asleep, being awake, or falling asleep. [16] Alpha, beta, theta, gamma, and delta waves are all seen in the different stages of sleep.
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 this regard, the ontogeny of the sleep cycle appears proportionate with metabolic processes, which vary in proportion with organism size. However, shorter sleep cycles detected in some elephants complicate this theory).
The term polyphasic sleep was first used in the early 20th century by psychologist J. S. Szymanski, who observed daily fluctuations in activity patterns. [2] It does not imply any particular sleep schedule. The circadian rhythm disorder known as irregular sleep-wake syndrome is an example of polyphasic sleep in
A 2023 study published in Sleep found that sleep regularity was a stronger predictor of overall mortality risk than sleep duration. Participants with higher SRI scores had as much as a 48% lower ...
The circadian rhythm provides a person with a signal for when to sleep and when to wake up. [43] If circadian rhythm and sleep-wake cycle are misaligned, this might lead to negative affect and emotional instability. [44] It has been found that emotions vary depending on the circadian rhythm and the duration of how long one was awake. [45]
After sustained periods of sleep, both the speed and synchronicity of the neurons firing are shown to decrease. [5] Another effect of wakefulness is the reduction of glycogen held in the astrocytes, which supply energy to the neurons. Studies have shown that one of sleep's underlying functions is to replenish this glycogen energy source. [6]
The appearances of these waves are most prominent in the period right before REM sleep, albeit they have been recorded during wakefulness as well. [1] They are theorized to be intricately involved with eye movement of both wake and sleep cycles in many different animals.