Sleep mechanisms. Regulation of the sleep-wake
In humans, the usual pattern is to stay awake for about 16 hours during the day and sleep eight hours coinciding with the nocturnal period. But sleep is regulated by individual, environmental and ontogenetic factors that widely vary in each individual. In conjunction with these special factors, homeostatic and circadian mechanisms influence our sleep regulation. The sleep-wake processes are regulated by the interaction between sleep homeostasis and circadian rhythm. The process affects the circadian timing of sleep and the homeostatic mechanisms governing the need for sleep.
Homeostasis was the main physiological regulatory mechanism of the body. It is defined as the set of phenomena of self-regulation, leading to the maintenance of relative constancy in the compositions and the properties of the internal environment of an organism. Homeostatic mechanisms, that control the bodys internal balance also govern sleep and waking times and the need for sleep is marked by homeostatic criteria. The homeostatic process, which promotes sleep, is low in the morning and act over the propensity of, sleep, which increases throughout the day and works the following way: when we need sleep, the "homeostatic"mechanism makes us feel sleepy and when we have slept enough, it acts to wake up. This mechanism maintains internal balance, so that more hours spent awake, the greater the need for sleep is, and sleeping longer hours due to its less intense. This need for sleep appearing in waking is regulated by substances such as adenosine, which is accumulated in the brain in proportion to the time spent in wakefulness.
Different animal species, including humans, organize many functions in periodic oscillations associated with variations in physiological parameters. When these fluctuations have a constant frequency, they are considered as biological rhythms generated by an endogenous mechanism of the body. J. Aschoff defined in 1981, that "biological rhythms include those events within a biological system, that occur more or less in regular intervals." These rhythms synchronize functions such as body temperature, heartbeat, hormone secretion and the endocrine system. When the oscillation of these variables follows a pattern closer to 24 hours it is called circadian rhythm. The sleep-wake cycle is the most evident in our circadian rhythms. This rate acts as a regulatory mechanism of sleep and works with chronometric properties similar to those of a clock. The body clock is governed by the endogenous or biological pacemaker pattern of activity and rest times.
The circadian regulation suposes, that regardless of the time spent in wakefulness, the need for sleep varies dependently, regardless of which time of day it is. In individuals with sleep wakefulness pattern it coincides with the day and night cycle, the most intense need for sleep band appears in the night from 3:00 to 4:00 in the morning, and in the range 14:00 to 15:00, where although drowsiness is less pronounced, the trend is also homeostatic sleep.
The mechanisms governing these cyclical phenomena, are being continuously studied for its remarkable importance in the field of chronobiology. Many physiological and behavioral functions in humans depend on these circadian variations. To adapt Circadian rhythms to the environment, the organism needs to receive environmental stimuli and external actors. The substantial anatomical basis of the circadian system is located in the suprachiasmatic nucleus of the hypothalamus, whose activity is influenced by various external stimuli, of which the most important is ambient light. In 1951 J. Aschoff coined the term "zeitgeber" synchronizers, referring to the environmental variables, that can affect the phases marked by internal clocks. These environmental synchronizers or markers complete the internal rhythmicity adapting it to the circumstances of the environment. The perfect cohesion between the two clocks, both internal and external, is the guarantee of the proper functioning of the body and proper expression of the sleep-wake cycle. Light is the most powerful "zeitgeber" and adults generally identify sleep-wake cycles with night and day (light and dark).