Sleep
Sleep is the process in which humans and other animals periodically rest, with greater or lesser degrees of unconsciousness and 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).
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2 Animal Sleep 3 Functions of Sleep 4 Sleep Disorders 5 Related Articles 6 Additional Resources |
Characteristics of Sleep
Sleep has two characteristics:
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
Human sleep are usually divided into 5 stages according to electroencephalographic (EEG) recordings:
- REM sleep with rapid eye movements
- 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
- Stage 2 with "splindles" (12-16Hz) and "K-complexes"
- Stage 3 with delta waves (1-2Hz) 20%-50% of the time.
- 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
Though there is still much debate about the evolutionary origins and purposes of sleep, it is widely theorised that one major function that occurs during sleep is consolidation and optimization of memories (including "unlearning").
Previously theorized functions that do not seem to be confirmed by research:
- 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
- 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
- conservation of energy.
Adenosine, a nucleoside which plays various roles in biochemical processes, gradually accumulates in the human brain during wakefulness but decreases during sleep.
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
A majority of sleep disorders which originate within the body (for example, insomnia, DSPS, ASPS) result from errors in synchronization of sleep with the body clock. Only a fraction of sleep problems are organic and cannot be resolved with chronotherapy. One of the simplest solutions towards getting good sleep is free-running sleep. Free-running sleep entails ignoring alarm clocks and schedules in order to sleep when, and only when tired. Free-running sleep can resolve the majority of synchronization-dependent sleep disorders, but usually cannot be employed due to the resulting loss of synchronization of sleep with the outside world (including day-night cycle).
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.