SLEEP PHYSIOLOGY

Sleep Physiology
Sleep is a brief state of unconsciousness in which the brain responds predominantly to internal rather than external stimuli. It is a self-regulating and easily reversible process.
Unlike other states of unconsciousness such as coma or general anesthesia, sleep is a natural, cyclic process that is self-regulated and easily reversible to wakefulness.


Electroencephalograms, or EEGs, are used to assess electrical activity in the cerebral cortex's superficial layers.
Different types of brain waves correspond to different levels of consciousness. High-frequency low-voltage beta-waves are produced by a fully awake and alert brain. Brain waves become slower in frequency and greater in voltage as consciousness fades.


Sleep is divided into two stages: rapid eye movement (REM) sleep and non-rapid eye movement (non-REM) sleep. Non-REM sleep occurs in three stages: N1 through N3 N1 is the condition of transition between awake and sleep. Alpha-waves dominate the EEG.
Light stimulation easily wakes the sleeping. N1 is usually only present for a few minutes. N2 is the following stage, a deeper sleep state in which stronger stimuli are required to awaken. Slower and more irregular brain activity with small bursts of "sleep spindles" and "K-complexes." Memory consolidation is thought to happen during this period.

N3 is greater in depth than N2. Delta-waves that are slow dominate. Muscles relax, vital signs drop, and rousing the sleeper is tough. Before REM sleep, N3 is often followed by a transition to N2. REM sleep is distinguished by rapid eye movements under the eyelids, as the term implies. It's also known as "paradoxical" sleep since the EEG of the brain is quite similar to that of awake.
Most dreams and autonomic reflexes occur during REM sleep. Vital signs are normal, but skeletal muscles are completely inhibited, preventing sleepers from acting out their dreams. This stage sequence occurs 4 to 5 times in a typical night. As the night progresses, the duration of N2 and REM sleep increases, while N3 decreases.

Sleep duration and timing are controlled by two key factors: homeostatic drive and circadian rhythm. The body's need for sleep, or the pressure to sleep, is referred to as homeostatic drive. It is at its lowest after a good night's sleep and begins to rise when we awaken. The desire to sleep will continue to grow until sleep is achieved.

Adenosine is understood to be a chemical that accumulates throughout awake hours and causes the pressure to sleep. Caffeine appears to increase wakefulness by acting as an antagonist to adenosine. Sleeping requirements rise with illness, as well as cognitively challenging or physically demanding activities.
Circadian rhythm is the body's circadian clock for the sleep-wake cycle. It regulates the time of sleep.

The master clock is housed in the hypothalamic suprachiasmatic nucleus, or SCN. It gets light input from the retina and resets the clock every day in accordance with the day-night cycle. The SCN is most active during the day and least active at night. The sleep-promoting area is found in the hypothalamic ventrolateral preoptic nucleus, VLPO. The SCN inhibits and adenosine activates the VLPO. GABA is used by the VLPO to inhibit wake-promoting areas of the brain, such as numerous nuclei in the reticular formation and the posterior hypothalamus.


The tuberomammillary nucleus, or TMN, and hypocretin neurons are two of these locations.
The TMN is mostly made up of histaminergic neurons, but it also generates GABA, which inhibits VLPO. The "switch" between sleep and wake is based on this mutual restraint. The hypocretin neurons stimulate the TMN and are essential for staying awake. Narcolepsy is caused by the loss of these neurons.

During the day, the SCN inhibits VLPO and stimulates hypocretin neurons, driving the switch towards waking. By the end of the day, when SCN activity is lowest and pressure to sleep is highest, VLPO is activated, and sleep is "switched" on.