July 6, 2022

Sleep (from the Latin somnus, with the same meaning) is an ordinary state of consciousness, complementary to the waking state (or *awake state), in which there is normal and periodic rest, characterized, both in humans and in other vertebrates, by the suspension of voluntary perceptual-sensory and motor activity. By saying complementary, in conjunction with ordinary, we only mean that, in most individuals (especially humans, here), such states of consciousness alternate, complementing each other ordinarily, periodically and regularly. . The sleep state is characterized by a typical brainwave pattern, essentially different from the pattern of the waking state, as well as that seen in other states of consciousness. Sleep, in this sense, means passing from the waking state to the sleeping state. In humans, the sleep cycle consisted of five stages, but currently there are four, as the N3 and N4 stages merged into N3, and last about ninety minutes (up to 120 minutes).[citation needed] ] It repeats itself four or five times during sleep. From what is recorded in the specialized literature, the longest period that a person has ever managed to go without sleep was eleven days. The purposes and mechanisms of sleep are still not entirely clear to science, but they are the subject of intense investigation.


Sleep can be defined as "a period of rest for body and mind, during which volition and consciousness are in partial or complete inactivity." Friedman (1795-1827) defined sleep as "the deliberate triggering of an alteration or reduction of the conscious state, which lasts a very long time, on average 8 hours (...) (…) generally resulting in a feeling of physical, psychic and intellectual energy restored". There are several definitions of sleep presented by different authors, and, in general, they complement each other. Sleep is important for health recovery in illness, while sleep deprivation can affect cell regeneration as well as the full recovery of immune function. The total number of hours of sleep for an adult person is normally between seven and eight hours.

The sleep cycle

A sleep cycle lasts about ninety minutes, with four to five cycles occurring in a nighttime sleep period. Eight hours is ideal for sleep. According to LAVIE (1998, 45), the number of cycles per night depends on the time of sleep, adding that "the sleep of a young person is usually composed of four or five of these cycles, with a tendency to decrease with age. advance of age". However, the common pattern varies between four to five cycles. During sleep, the individual goes through, usually repetitively, cycles, starting with stage N1 of NREM sleep, progressing to stage N3, regressing to stage N2, and entering REM sleep. It returns to stage N2 again and the entire cycle is repeated again. In the first sleep cycles, NREM periods (specifically the N3 stage) are longer than REM. As sleep progresses, the N3 stage begins to shorten and the REM period begins to lengthen. In the first part of sleep, REM sleep predominates, with the REM periods being longer in the second half. Sleep, as opposed to wakefulness (stage W) is divided into two physiologically distinct types: NREM (Non Rapid Eye Movement or "Non Rapid Eye Movement"); and REM (Rapid Eye Movement)

NREM sleep

NREM (or non-REM) sleep occupies about 75% of sleep time and is divided into three distinct periods known as stages N1, N2, and N3. Stage N1: Starts with a drowsiness. It lasts approximately five minutes. The person falls asleep. It is characterized by an EEG similar to that of the waking state.