Sleep is a naturally recurring altered state of consciousness with relatively suspended sensory and motor activity, characterized by the inactivity of nearly all voluntary muscles.[1] It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, but it is more easily reversible than hibernation or coma. Sleep is a heightened anabolic state, accentuating the growth and rejuvenation of the immune, nervous, skeletal and muscular systems. It is observed in all mammals, all birds, and many reptiles, amphibians, and fish.
The purposes and mechanisms of sleep are only partially clear and are the subject of intense research.
The purposes and mechanisms of sleep are only partially clear and are the subject of intense research.
In mammals and birds, sleep is divided into two broad types: rapid eye movement (REM) and non-rapid eye movement (NREM or non-REM) sleep. Each type has a distinct set of associated physiological, neurological, and psychological features. The American Academy of Sleep Medicine (AASM) further divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or slow-wave sleep (SWS).[3
Stage N1 refers to the transition of the brain from alpha waves having a frequency of 8 to 13 Hz (common in the awake state) to theta waves having a frequency of 4 to 7 Hz. This stage is sometimes referred to as somnolence or drowsy sleep. Sudden twitches and hypnic jerks, also known as positive myoclonus, may be associated with the onset of sleep during N1. Some people may also experience hypnagogic hallucinations during this stage, which can be troublesome to them. During N1, the subject loses some muscle tone and most conscious awareness of the external environment.
Stage N2 is characterized by sleep spindles ranging from 11 to 16 Hz (most commonly 12–14 Hz) and K-complexes. During this stage, muscular activity as measured by EMG decreases, and conscious awareness of the external environment disappears. This stage occupies 45% to 55% of total sleep in adults.
Stage N3 (deep or slow-wave sleep) is characterized by the presence of a minimum of 20% delta waves ranging from 0.5 to 2 Hz and having a peak-to-peak amplitude >75 μV. (EEG standards define delta waves to be from 0 – 4 Hz, but sleep standards in both the original R&K, as well as the new 2007 AASM guidelines have a range of 0.5 – 2 Hz.) This is the stage in which parasomnias such as night terrors, nocturnal enuresis, sleepwalking, and somniloquy occur. Many illustrations and descriptions still show a stage N3 with 20%-50% delta waves and a stage N4 with greater than 50% delta waves; these have been combined as stage N3.
[edit] REM sleep
Main article: Rapid eye movement sleep
Rapid eye movement sleep, or REM sleep, accounts for 20%–25% of total sleep time in most human adults. The criteria for REM sleep include rapid eye movements as well as a rapid low-voltage EEG. Most memorable dreaming occurs in this stage. At least in mammals, a descending muscular atonia is seen. Such paralysis may be necessary to protect organisms from self-damage through physically acting out scenes from the often-vivid dreams that occur during this stage.
Stage N2 is characterized by sleep spindles ranging from 11 to 16 Hz (most commonly 12–14 Hz) and K-complexes. During this stage, muscular activity as measured by EMG decreases, and conscious awareness of the external environment disappears. This stage occupies 45% to 55% of total sleep in adults.
Stage N3 (deep or slow-wave sleep) is characterized by the presence of a minimum of 20% delta waves ranging from 0.5 to 2 Hz and having a peak-to-peak amplitude >75 μV. (EEG standards define delta waves to be from 0 – 4 Hz, but sleep standards in both the original R&K, as well as the new 2007 AASM guidelines have a range of 0.5 – 2 Hz.) This is the stage in which parasomnias such as night terrors, nocturnal enuresis, sleepwalking, and somniloquy occur. Many illustrations and descriptions still show a stage N3 with 20%-50% delta waves and a stage N4 with greater than 50% delta waves; these have been combined as stage N3.
[edit] REM sleep
Main article: Rapid eye movement sleep
Rapid eye movement sleep, or REM sleep, accounts for 20%–25% of total sleep time in most human adults. The criteria for REM sleep include rapid eye movements as well as a rapid low-voltage EEG. Most memorable dreaming occurs in this stage. At least in mammals, a descending muscular atonia is seen. Such paralysis may be necessary to protect organisms from self-damage through physically acting out scenes from the often-vivid dreams that occur during this stage.
Sleep timing is controlled by the circadian clock, sleep-wake homeostasis, and in humans, within certain bounds, willed behavior. The circadian clock—an inner timekeeping, temperature-fluctuating, enzyme-controlling device—works in tandem with adenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine is created over the course of the day; high levels of adenosine lead to sleepiness. In diurnal animals, sleepiness occurs as the circadian element causes the release of the hormone melatonin and a gradual decrease in core body temperature. The timing is affected by one's chronotype. It is the circadian rhythm that determines the ideal timing of a correctly structured and restorative sleep episode.[11]
Optimal amount in humans
[edit] Adult
The optimal amount of sleep is not a meaningful concept unless the timing of that sleep is seen in relation to an individual's circadian rhythms. A person's major sleep episode is relatively inefficient and inadequate when it occurs at the "wrong" time of day; one should be asleep at least six hours before the lowest body temperature.[16] The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening:[17]
maximum concentration of the hormone melatonin, and
minimum core body temperature.
Human sleep need can vary by age and among individuals, and sleep is considered to be adequate when there is no daytime sleepiness or dysfunction. Moreover, self-reported sleep duration is only moderately correlated with actual sleep time as measured by actigraphy,[18] and those affected with sleep state misperception may typically report having slept only four hours despite having slept a full eight hours.[19]
[edit] Adult
The optimal amount of sleep is not a meaningful concept unless the timing of that sleep is seen in relation to an individual's circadian rhythms. A person's major sleep episode is relatively inefficient and inadequate when it occurs at the "wrong" time of day; one should be asleep at least six hours before the lowest body temperature.[16] The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening:[17]
maximum concentration of the hormone melatonin, and
minimum core body temperature.
Human sleep need can vary by age and among individuals, and sleep is considered to be adequate when there is no daytime sleepiness or dysfunction. Moreover, self-reported sleep duration is only moderately correlated with actual sleep time as measured by actigraphy,[18] and those affected with sleep state misperception may typically report having slept only four hours despite having slept a full eight hours.[19]
Age and condition vs Average amount of sleep per day
Newborn: up to 18 hours
1–12 months: 14–18 hours
1–3 years: 12–15 hours
3–5 years: 11–13 hours
5–12 years: 9–11 hours
Adolescents: 9–10 hours[29]
Adults, including elderly: 7–8(+) hours
Pregnant women: 8(+) hours
Sleep debt is the effect of not getting enough rest and sleep; a large debt causes mental, emotional, and physical fatigue.
Sleep debt results in diminished abilities to perform high-level cognitive functions. Neurophysiological and functional imaging studies have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.[30]
Scientists do not agree on how much sleep debt it is possible to accumulate; whether it is accumulated against an individual's average sleep or some other benchmark; nor on whether the prevalence of sleep debt among adults has changed appreciably in the industrialized world in recent decades. It is likely that children are sleeping less than previously in Western societies.[31]
Sleep debt results in diminished abilities to perform high-level cognitive functions. Neurophysiological and functional imaging studies have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.[30]
Scientists do not agree on how much sleep debt it is possible to accumulate; whether it is accumulated against an individual's average sleep or some other benchmark; nor on whether the prevalence of sleep debt among adults has changed appreciably in the industrialized world in recent decades. It is likely that children are sleeping less than previously in Western societies.[31]
In the 1950s, a radio disc jockey by the name of Peter Tripp engaged on what was (and still is), a popular radio amusement, the "stunt." (Nowadays, it's just about the norm for wacky antics to pervade radio stations across the nation.)
In Peter's case, his objective was to break the world's record for staying awake. He didn't sleep for days and days while he played records and bantered with his audience from a glass booth in Times Square. He was on the air for about 3 hours per day. The rest of the time, he was kept awake by nurses who nudged him when he began to drop off. Doctors and scientists who were studying him also played games with him, asked him questions continually, and did everything they could to keep his attention.
In all, he was able to stay awake for a total of 201 hours before he simply passed out. That's over eight days! But it came with a price.
During the wake-a-thon, and after only a couple of days, Peter Tripp began to hallucinate. He saw cobwebs on his shoes. He saw mice and kittens that weren't there. He rummaged through drawers looking for money that never existed. He also accused a technician of dropping a hot electrode into his shoe. Basically, he had totally "flipped out."
When he finally ended the ordeal, he went to sleep for over 13 hours.
In Peter's case, his objective was to break the world's record for staying awake. He didn't sleep for days and days while he played records and bantered with his audience from a glass booth in Times Square. He was on the air for about 3 hours per day. The rest of the time, he was kept awake by nurses who nudged him when he began to drop off. Doctors and scientists who were studying him also played games with him, asked him questions continually, and did everything they could to keep his attention.
In all, he was able to stay awake for a total of 201 hours before he simply passed out. That's over eight days! But it came with a price.
During the wake-a-thon, and after only a couple of days, Peter Tripp began to hallucinate. He saw cobwebs on his shoes. He saw mice and kittens that weren't there. He rummaged through drawers looking for money that never existed. He also accused a technician of dropping a hot electrode into his shoe. Basically, he had totally "flipped out."
When he finally ended the ordeal, he went to sleep for over 13 hours.
Scientists have shown numerous ways in which sleep is related to memory. In a study conducted by Turner, Drummond, Salamat, and Brown,[42] working memory was shown to be affected by sleep deprivation. Working memory is important because it keeps information active for further processing and supports higher-level cognitive functions such as decision making, reasoning, and episodic memory. The study allowed 18 women and 22 men to sleep only 26 minutes per night over a four-day period. Subjects were given initial cognitive tests while well-rested, and then were tested again twice a day during the four days of sleep deprivation. On the final test, the average working memory span of the sleep-deprived group had dropped by 38% in comparison to the control group.
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