The Physiology of Sleep

Sleep is a natural state in which the body rests and conserves energy. Humans experience regular episodes of sleep. During this time, the brain becomes less responsive to external stimuli, allowing it to grow and repair tissues while conserving energy. The stages of sleep include deep, REM, melatonin, light, and rapid eye movement (RWMA). The five-stage cycle repeats approximately five times during a single episode of sleep. In infants, sleep periods may last up to six or seven hours, with morning and afternoon naps. In children, the cycle may consist of one to three-hour long REM periods.


Research has shown that sleep is essential for optimal health and well-being. It helps regulate metabolic, immune, and thermoregulation, as well as brain maturation, synaptic optimization, and learning. The mechanism of sleep is complex, involving genes, neurotransmitters, and circadian rhythms. Adult humans experience different types of sleep, varying durations, and are more sensitive to certain types of light and sound. In addition to age and sex, different levels of sleep also differ in length and intensity.

Although the physiological nature of sleep has long fascinated researchers, it remains a mystery. The brain does not appear to exhibit the same patterns as those of non-human animals, which is why it is important to understand how humans differ from them. However, recent advances in animal experimentation and computer modeling of EEG signals have made it possible to study the physiological activity of the brain while the subject is asleep. This information can aid scientists in identifying specific structures involved in the process.

Molecular mechanisms of sleep vary. Several experimental studies have linked physiological variables with behavioral sleep. In addition to regulating body temperature and metabolism, sleep is also thought to stimulate the immune system, detoxify waste, and improve synaptic connections. The definition of sleep is a complex phenomenon and is largely based on multiple criteria. The physiological characteristics of sleep differ among different species, and there are variations in their duration in humans. The most reliable definitions of sleep are based on agreement between two or more observers.

The brain undergoes two different stages during sleep: REM sleep and NREM. The body cycles through these two stages of the night four to six times, lasting 90 minutes each. At night, the brain is in a deep phase, and neither muscle nor eye movements are noticeable. The REM phase is the most intense, and the brain’s activity is influenced by the stage of the sleep. The NREM stage has the highest concentrations of activity.

There are no single measurable physiological factors that define sleep. Instead, the body’s electrical activity is regulated during sleep. In fact, there are two major types of sleep: deep and REM. Both have distinct physiological functions. REM is a period of complete inactivity, while REM sleep is a period of deep rest. For humans, it’s a state of consciousness in which all the functions of the body are active and functioning normally.