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Sleep

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Sleep is the process in which humans and other animals periodically rest, with decreased responsiveness to the surrounding world. Sleep occurs cyclically, roughly every 24 hours even though the average human inner body clock usually runs a 24.5-25.5 hour cycle. This cycle gets reset daily (to match 24 hours) with various stimuli such as sunlight. One of the correlates of this cycle is the level of melatonin, which is high at times when we tend to sleep. Some people sleep twice every 24 hours (afternoon nap, siesta).

Characteristics of Sleep

1. It is reversible. A sleeping person can be awakened.
2. During it, one loses consciousness of one's surroundings. The sleeper is not directly aware of the outside world (although, for example, sounds can be incorporated into dreams).

These two characteristics separate real sleep from many things that are described as "sleep", but really are not, such as coma or death.

Stages of sleep

1. REM sleep with rapid eye movements
2. Stage 1 with 50% reduction in alpha waves compared to awake resting with eyes closed. The stage is sometimes referred to as somnolence or "drowsy sleep". It appears at sleep onset and can be associated with so-called hypnagogic hallucinations
3. Stage 2 with "splindles" (12-16Hz) and "K-complexes"
4. Stage 3 with delta waves (1-2Hz) 20%-50% of the time.
5. Stage 4 with delta waves over 50% of the time

Animal Sleep

Animals vary widely in their amounts of sleep, from 2 hours a day for giraffes to 20 hours for bats. Cats are one of the few animals that do not have most of their sleep consolidated into one session, preferring instead to spread their sleep fairly evenly throughout the day.

Seals and dolphins "sleep" with alternate hemispheres of their brains asleep and the other awake. Seals need to do this so they can breathe above water while sleeping.

Even fish and fruit flies appear to sleep. If fruit flies are repeatedly disturbed so that they can not sleep, later when allowed to sleep they will stay inactive for a longer period of time.

Many animals hibernate in a deep sleep during winter to save warmth and energy. A similar kind of sleep is estivation, which is hibernating to escape the heat of summer.

Functions of Sleep

• promotion of physiological processes which rejuvenate the body and the mind: some studies suggest sleep restores neurons and increases production of brain proteins and certain hormones;

• unlearning during sleep prevents the brain from becoming overloaded with knowledge; and

• avoidance of danger: prehistoric mankind adapted the pattern of sleeping in caves at night, because it protected humans from species physiologically suited to function well in the dark, such as saber-toothed cats.

Sleep proceeds in cycles of NREM and REM phases. Each phase has a distinct physiological function. Dreaming, for example, appears to occur during REM sleep.

Adenosine, a nucleoside which plays various roles in biochemical processes, gradually accumulates in the human brain during wakefulness but decreases during sleep. The precise reasons and implications of this are not yet known, however.

Some medications (for example, sleeping pills) can suppress selective stages of sleep. This can result in obtaining sleep (loss of consciousness) without fulfilling its physiological function (memory remolding).

Sleep Disorders

Sleep disorder is often observed in patients with a number of psychiatric problems (e.g. bipolar disorder, depression, schizophrenia, etc.).

One form of sleep disorder, narcolepsy, probably has a genetic basis. Subjects not only fall asleep several times during the day, they also experience abnormal sleep patterns at night. A new medication, recently been approved by the FDA, is Xyrem. This is the proprietary name of gamma-hydroxybutyrate (GHB).

Sleep can also refer to the state of hypnosis.

Facts

• Often times, the lips are most responsive when met with another pair of lips. This will sometimes cause the sleeper to jolt suddenly, as maybe a defense system.

Related Articles

• Dreaming
• Seasonal Affective Disorder
• Sleep disorders
  • Bruxism
  • Narcolepsy
  • Sleep apnea
  • Sleep paralysis
  • Sleepwalking
  • Snoring
• Lodging, list of types of lodging, bed, sleeping bag, sleeping pad
• Yawn
• Myclonic twitch
• Sleep debt
• Power nap
• Awake

Additional Resources

• Animal Sleep
• More about animal sleep
• Stories about the sleep of famous people

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