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Sleeping is a state characterised by the suspension of consciousness, loss of response to external stimuli and decreased motor activity. Normally, the eyes are closed and a typical supported posture adopted.  

Sleeping is a function of the nervous system notable for being natural, necessary, periodic, cyclical and reversible. It is universal in the animal kingdom. It is essential for life, since its absolute deprivation leads to death in less than four weeks.  

Sleep consists of a complex network of neurological mechanisms that converge in a state where the energy and metabolic needs of the brain do not disappear, but are maintained and may even be higher than required in the daytime (in some phases of sleep and in some regions of the brain). Sleep is, therefore, a dynamic state in which neurons are still active and performing functions different from those conducted during the day.

Sleep is a function of the nervous system; it is a set of cells that perform a periodic and transitory function essential for life.

Stages of sleep

Sleep is divided into two main phases:  

Non-REM phase: This phase has no rapid eye movements (REM). This phase itself is divided into 3 states: N1 (drowsiness, 10% of the time), N2 (light sleep, 45% of the time), and N3 (deep sleep, 20% of the time).  

REM phase: This phase has rapid eye movements (REM). This is deep sleep phase in which the brain is very active. The brain stem blocks the motor neurons so the person cannot move. In this phase you dream. REM sleep is considered to be involved in the process of memory and learning. 

In a young healthy adult, non-REM sleep occupies approximately 75% of sleep time and REM the remaining 25%.

Substantiated information by:

Alejandro Iranzo de Riquer

Published: 26 May 2022
Updated: 15 June 2022

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