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Sleep can follow a physiological or behavioral definition. In the physiological sense, sleep is a state characterized by reversible unconsciousness, special brainwave patterns, sporadic eye movement, loss of muscle tone (possibly with some exceptions; see below regarding the sleep of birds and of aquatic mammals), and a compensatory increase following deprivation of the state, this last known ...
Children need many hours of sleep per day in order to develop and function properly: up to 18 hours for newborn babies, with a declining rate as a child ages. [65] Early in 2015, after a two-year study, [ 91 ] the National Sleep Foundation in the US announced newly revised recommendations as shown in the table below.
As sleep time decreased over time from the 1950s to 2000s from about 8.5 hours to 6.5 hours, there has been an increase in the prevalence of obesity from about 10% to about 23%. [ 2 ] Weight gain itself may also lead to a lack of sleep as obesity can negatively affect quality of sleep, as well as increase risk of sleeping disorders such as ...
The National Sleep Foundation recommends that adults aim for 7–9 hours of sleep per night, while children and teenagers require even more. For healthy individuals with normal sleep, the appropriate sleep duration for school-aged children is between 9 and 11 hours.
In 1896, Patrick and Gilbert observed that during a prolonged period of sleep deprivation, sleepiness increases and decreases with a period of approximately 24 hours. [10] In 1918, J.S. Szymanski showed that animals are capable of maintaining 24-hour activity patterns in the absence of external cues such as light and changes in temperature.
The number of hours of sleep is variable, however the proportion of sleep spent in a particular stage remains mostly consistent; healthy adults normally spend 20–25% of their sleep in REM sleep. [5] During rest following a sleep-deprived state, there is a period of rebound sleep which has longer and deeper episodes of SWS to make up for the ...
During a night of sleep, humans usually experience about four or five periods of REM sleep; they are shorter (~15 min) at the beginning of the night and longer (~25 min) toward the end. Many animals and some people tend to wake, or experience a period of very light sleep, for a short time immediately after a bout of REM.
Schematic illustration of a normal sleep cycle. 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.