Sleep is divided into two broad types: rapid eye movement (REM sleep) and non-rapid eye movement (non-REM or NREM sleep). REM and non-REM sleep are so different that physiologists classify them as distinct behavioral states. Dreams (or nightmares) occur during REM sleep. REM sleep is associated with esynchronized and fast brain waves, loss of muscle tone and suspension of homeostasis[citation needed]. NREM is considered to be deep sleep (the deepest part of NREM is called slow wave sleep); it shows no prominent eye movement or muscle paralysis.

Sleep occurs in cycles of approximately 90 minutes. This rhythm is called the ultradian sleep cycle. Sleep proceeds in cycles of NREM and REM, normally in that order and usually four or five such cycles per night. The American Academy of Sleep Medicine (AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or slow-wave sleep.The whole period normally proceeds in the order: N1 → N2 → N3 → N2 → REM. REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep. An adult reaches REM approximately every 90 minutes; REM sleep usually lasts for longer during latter half of sleep than in the early part of the sleep episode. There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening.

Sleep is a naturally recurring state of mind and body characterized by altered consciousness, relatively inhibited sensory activity, inhibition of nearly all voluntary muscles, and reduced interactions with surroundings. It is distinguished from wakefulness by a decreased ability to react to stimuli, but is more easily reversed than the state of being comatose. Sleep occurs in repeating periods, in which the body alternates between two highly distinct modes known as non-REM and REM sleep. Although REM stands for "rapid eye movement", sleep affects other brain-body functions, including virtual paralysis of the body.

Sleep has a large impact on the respiratory system, and vice versa. Ventilation (entry and exit of air into the lungs) and respiration (transportation of oxygen into circulation in the lungs, and of carbon dioxide in the opposite direction) both change while a person sleeps. Specifically, they become faster and more erratic during rapid eye movement (REM) sleep. The cough reflex is also suppressed during REM and NREM sleep.

In people with sleep-disordered breathing, respiration pauses often occur during sleep due to the airways either completely or partially collapsing. Obstructive sleep apnea (OSA, in which no ventilation occurs) and hypopneas (reduced ventilation due to partial airway obstruction) lead to intermittent and abrupt reduction in blood oxygen levels. These bring individuals to a more awake stage of sleep and can wake them up. Sleep-disordered breathing symptoms include snorting, snoring, gasping, and choking in one’s sleep (often these occur without the sleeper’s awareness). They result in excessive daytime sleepiness (or “tiredness”); often, the individual has no idea whey they are so tired.

This respiratory system interruption also has other, negative, long-term results, including increased activation of the adrenalin-secreting sympathetic nervous system, inflammation, and hormonal changes. These, in turn, can increase the risk of hypertension, coronary artery disease (blocked heart arteries and heart attacks), abnormal heart rhythms (like atrial fibrillation), stroke, a rise in blood sugars (glucose intolerance) to the point of diabetes, obesity, mood problems (such as depression, memory problems), and other issues. In fact, OSA significantly increased the risk of death from any cause — independent of other variables ..

Sleep has been considered a restorative or a recovery phase that prepares the body for the next episode of wakefulness. Cell division is more rapid during non-REM sleep and sleep has an important function on the immune system.

H umans spend about one-third of their lives asleep. ... Circadian rhythms, the daily rhythms in physiology and behavior, regulate the sleep-wake cycle. In addition, thesleep-wake system is thought to be regulated by the interplay of two major processes, one that promotes sleep and one that maintains wakefulness