Circadian Rhythms

Living things go through physical states linked to the 24-hour cycle of light and darkness. In humans, these changes influence strength, alertness, temperature, and hormone levels.
Circadian rhythms are cyclical biological changes that take place on a roughly 24-hour schedule. These daily rhythms include cycles between sleepiness and alertness. These rhythms are controlled by a "biological clock" in the brain that consists of about 20,000 neurons located in the hypothalamus, a part of the brain controlling thirst, fatigue, and certain functions of the autonomic nervous system. These neurons, which transmit messages between the brain and body, receive direct input from the eyes, allowing them to coordinate body systems based on light input. Light can turn on and off genes that control the molecular structure of these neurons. As night approaches, neurons send a signal to the pineal gland in the brain to release melatonin, a hormone that triggers a feeling of sleepiness. Melatonin production usually peaks between midnight and 3:00 a.m. Changes in light-dark cycles (such as reading under bright lights at night or looking at a bright computer screen) can speed up, slow down, or reset circadian rhythms.
Circadian rhythms control physical changes that run on a roughly 24-hour cycle.
Other hormones are also coordinated by the brain's biological clock. Acetylcholine and glucocorticoids, such as cortisol, are produced by the adrenal cortex and peak during the morning. These hormones cause people to feel awake and energized. The biochemical process of metabolism is also regulated by the brain's biological clock. For example, insulin helps regulate blood sugar levels. Insulin secretion reaches its highest levels around 5:00 p.m. and falls to its lowest levels around 4:00 a.m. If insulin levels drop too low, the person usually wakes up and has trouble falling back to sleep.

Disruptions and poor regulation of circadian rhythms can increase inflammation and the risk for developing metabolic, endocrine, cardiovascular, and tumor-causing diseases.

Sleep Stages

Sleep consists of multiple stages that repeat in 60- to 90-minute cycles. Dreams occur mostly during rapid eye movement (REM) sleep, which is characterized by rapid eye movements and heightened brain activity.
Sleep is not a uniform experience. Instead, people cycle through different stages of sleep several times each night. Sleep stages can be monitored with electroencephalograms (EEGs), tests that track electrical activity in the brain. Most dreams occur during rapid eye movement sleep, or REM sleep, which is characterized by fast eye movements. EEG activity during REM sleep resembles EEG activity when awake, but the motor cortex (which controls the body's muscles) is turned off to prevent movement. In REM sleep, the person enters a virtual world of imagery. This stage allows people to move about in the dream world while their body remains motionless. The other sleep stages are referred to as non-REM sleep, or NREM sleep, in which rapid eye movements do not occur. Some models break NREM sleep into four stages, called Stage 1 through Stage 4. Other models divide NREM sleep into three stages, called NREM1 through NREM3 or N1 through N3. These models consider both Stage 3 and Stage 4 sleep part of NREM3.

Stage 1 (or NREM1) is the lightest stage of sleep. EEG recordings show brain waves that are slightly slower than (but similar to) those seen during wakefulness. People disturbed during this stage of sleep may not even recognize that they had fallen asleep. Stage 2 sleep (or NREM2) is characterized by bursts of oscillating brain activity. This activity produces a pattern on the EEG called a sleep spindle. When people enter this stage of sleep, they are more difficult to wake. Stages 3 and 4 sleep (or NREM3) are the deepest, most physically restorative stages. These stages are referred to as slow-wave sleep because EEG recordings show very slow, synchronized waves. Tissue repair occurs during these stages. Growth hormone is also secreted to repair muscles. The brain's glucose levels are also restored to fuel its activity. Slow-wave sleep is necessary for memory consolidation, which helps individuals process and retain information in long-term memory.

The sleep cycle begins with Stage 1 sleep followed by Stages 2, 3, and 4. The stages are then reversed as sleep becomes lighter. Each cycle culminates in brief periods of REM sleep. Then the cycle repeats, with each full sleep cycle lasting about 60–90 minutes. During the first hours of sleep, people spend more time in the deeper stages. As sleep progresses, people spend more time in the lighter stages and in REM sleep.

Sleep Stages

Each night, people cycle repeatedly through multiple stages of sleep, from light sleep (Stages 1 and 2, also called NREM1 and NREM2) to deep sleep (Stages 3 and 4, also called NREM3). REM sleep involves dreaming.

Health Impacts of Sleep Deprivation

Sleep deprivation has many negative impacts, including reduced immune function, impaired cognition, memory disruption, hormonal disruption, and weight gain.
Sleep is necessary for physical and mental health. Sleep deprivation has many negative health impacts, including reduced immune function, impaired cognition, memory disruption, hormonal disruption, and weight gain. Newborns require about 16 hours of sleep daily. The average six-year-old needs about 11 to 12 hours every day. Adults require seven to nine hours on average to function well. Individuals tend to need less sleep as their physical and mental development completes.

Health Risks Associated with Sleep Deprivation

Sleep deprivation causes negative effects throughout the body. It impairs thinking and memory, reduces thyroid function (hypothyroidism), disrupts the immune system, and increases the risk of disease.
Three theories have been proposed to explain why sleep (or lack of it) greatly influences physical and mental health. According to the repair theory of sleep, sleep is necessary for restoring physiological and mental functions. The brain uses sleep to flush out waste that is toxic to brain cells. Selectively depriving people of specific sleep stages (by waking them when they enter them) leads to mental and physical problems. A lack of REM sleep results in poor procedural memory and increased aggressiveness. It also leads to REM rebound, in which people spend more time in REM sleep when they are finally allowed to sleep normally. Depriving people of deep, slow-wave sleep stages, which are the most restorative, makes people feel extremely tired. It also causes hypersensitivity to muscle and joint pain and impairs storage of memories for facts.

The adaptive theory of sleep (also called inactivity theory) proposes that inactivity during the day- or nighttime is an adaptation for survival. Sleep may help animals avoid periods of oppressively high temperatures (day sleepers) or low visibility (night sleepers). Animals that stay quiet at night are more likely to survive predation. A survival advantage is also conferred on those who remain conscious enough to fight or flee.

Energy conservation theory proposes that sleep evolved to reduce energy demands and expenditure. Sleep can reduce energy metabolism during periods of the day or night that present few opportunities to obtain food.

Sleep Disorders

Insomnia, sleep apnea, and narcolepsy are common causes of sleep deprivation. Sleepwalking, night terrors, and sleep paralysis are caused by transitional glitches between sleep states or between sleep and waking states.
The most common sleep disorders are insomnia, sleep apnea, restless legs syndrome, and narcolepsy. Insomnia is a disorder in which a person has a hard time falling or staying asleep. Insomnia can be acute (lasting for days or weeks) or chronic (lasting longer than a month). Common causes of acute insomnia include stress, family conflict, and trauma. Most cases of chronic insomnia are secondary, meaning they are a symptom or a side effect of some other medical condition. These may include Alzheimer's and Parkinson's disease, depression, anxiety, post-traumatic stress disorder, gastrointestinal disorders, and an overactive thyroid (also known as hyperthyroidism). Certain medications can produce chronic insomnia, including those for asthma, allergies, and certain heart and blood pressure conditions. Overconsumption of caffeine, tobacco, and alcohol can also cause chronic insomnia.

Sleep apnea refers to a sleep disorder in which breathing interruptions occur frequently during sleep. There are two types of sleep apnea: obstructive sleep apnea and central sleep apnea. In obstructive sleep apnea, the airway collapses or becomes blocked during sleep. People will often snore loudly or awake with a start, feeling like they are suffocating. Being overweight, being male, having small airways, and having a family history of sleep apnea increase risk. Central sleep apnea occurs because the brain does not send proper signals to the muscles that control breathing. Both types of sleep apnea can be controlled with a breathing device called a continuous positive airway pressure (CPAP) machine.

Restless legs syndrome (RLS) involves a powerful urge to move the legs when lying down, sitting, or falling asleep. People with RLS often have a condition called periodic limb movement disorder (PLMD), in which their arms or legs twitch uncontrollably during sleep. Asthma and pregnancy increase the risk of RLS, as can caffeine, tobacco, and alcohol. In most cases, however, the cause is unknown.

Narcolepsy is a chronic neurological disorder affecting the brain's ability to control sleep-wake cycles. During the day, a narcoleptic person may suddenly fall asleep, even while driving, eating, or talking. People with narcolepsy may also suffer sudden muscle weakness that causes them to collapse or be unable to move. Some narcoleptics also experience vivid dreamlike images while awake (hallucinations). Anywhere from 135,000 to 200,000 people in the United States have narcolepsy, with men and women equally affected. People with narcolepsy often experience uneven and interrupted sleep. They may wake frequently during the night or suffer total paralysis just before falling asleep or just after waking up, known as sleep paralysis. People without narcolepsy may also experience sleep paralysis.

Other common sleep behaviors include night terrors and sleepwalking. Night terrors are episodes of screaming, intense fear, and flailing while asleep. Almost 40 percent of children experience sleep terrors. Most children outgrow them by adolescence, and adults rarely experience them. Although these experiences can be quite frightening, they are not usually cause for concern. People who experience night terrors will not remember them the next day. In sleepwalking, people get up and walk around while asleep. This disorder is more common in children than adults and usually resolves by the teen years.

Night terrors, sleepwalking, and sleep paralysis seem to involve faulty transitions between sleep stages. Some parts of the brain are in one stage of sleep, while others are awake and active. People's bodies should be paralyzed during REM sleep to keep them from physically enacting dreams. In sleep paralysis, that protective paralysis engages too soon or disengages too slowly, leaving people conscious but unable to move. In sleepwalking, people walk, talk, and eat while sleeping because they are not paralyzed when they should be.