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Sleeping is associated with a state of muscle relaxation and limited perception of environmental stimuli.

Sleep is a state of rest, which happens in animals, including humans. Animals that sleep are in an unconscious state, or mostly so. Most of the muscles that animals can control on purpose are inactive.[1] Animals that are asleep do not react to stimuli as fast as those that are awake. They can wake up from sleep more easily than from hibernation or coma. All mammals and birds, and many reptiles, amphibians, and fish, sleep. In humans, other mammals, and most other animals that were studied, sleeping regularly is essential for survival.

The purpose of sleep is not fully known, and it is still studied today.[2] We do know for sure, though, that sleep is necessary to most animals for survival.[3] Not having enough sleep can even be used for torture.

A person sleeps when he or she is not awake. Usually, this is during the night. During the day, most people are awake. They work, go to school or university, or do other things. Many people sleep for a short time in the early afternoon for a quick rest. This is often called a nap.

In some countries, most notably where the weather is warm, there is a tradition to take a nap right after noon, or early in the afternoon. This tradition is called siesta, and is most notable in Spain and Latin America. Some stores and services even have closed while their owners and/or employees take their siesta.


The word "sleep" comes from the old Old Germanic verbs for sleep.[4] In Old and Middle High German it was called "SLAF". The original meaning of the word was "to slap", which was related to the word for "flabby" (not hard or firm).[5]

Many words related to "sleep" have very different meanings, though. For example, "sleep" was also used to mean death, so that "putting an animal to sleep" meant to kill the animal without pain.[4] "Sleep with someone" can also have a [[|sexual]] meaning.[4]

What sleep is for

Generally, the reason for sleep is that the brain has work to do during sleep. The details are not fully understood, but it is important to get enough sleep for the body and the brain to be healthy and work properly. Usually animals (and people) sleep at periodic intervals, such as once a day. Certain animals send out signals to the others that they will soon go to sleep. Yawning is such a signal.

Both humans and many animals sleep about once a day. Some animals, such as cats, sleep many times a day for short periods.

When people sleep they often have dreams. Probably some animals do, too.

Not only people sleep, all mammals and birds, and most fish, reptiles and other animals do, too.

Different categories of sleep

In mammals and birds, sleep can be divided into two categories. In one of them, the eyes move rapidly. It is called REM-sleep (from rapid eye movement). Most dreams take place in this phase. REM-sleep occurs normally at intervals throughout the night, and the periods of REM-sleep increase in length in the second half of the night. REM-sleep was first discovered in 1952-53.

The other category, where this movement of the eyes does not happen, is called NREM-sleep (Non-REM sleep). Usually, dreams do not occur during this time. There are three or four stages of NREM-sleep. Stage I is just barely sleeping, or dozing. Stage II is also light sleep. Normally, in adult humans, about half of the time spent asleep is spent in light sleep. Stages III and IV are called deep sleep. Deep sleep is necessary for growth and healing. It can be quite difficult to awaken someone who is in stage III or stage IV sleep. Sometimes stages III and IV are combined and called stage III.

Adult humans normally sleep in cycles of 90 to 110 minutes each. The night's sleep can be 4 or 5 of these cycles. Each cycle includes, in this order: stage I, stage II, stage III (IV), stage II and REM.

Getting enough sleep

The timing of sleep and the amount of it are both important. Both are different for different people. Some adults sleep best from 22:00 to 05:00 or 06:00 or 07:00. Some sleep best from midnight to seven or eight. These variations are normal.

How much sleep is enough also depends on age. Children need more sleep than adults. Newborn babies sleep about 18 hours per day. Small babies sleep many times a day; human babies do not develop circadian rhythms before they are 3 – 4 months old. At the age of 1 year, they sleep for about 14 hours.

A nine-year-old should sleep about 9–10 hours per day and teenagers, too, need that much sleep. Adults who sleep less than about 8 hours a day perform worse than those who sleep that long. [6][7]

Sleeping problems

People may have trouble going to sleep, staying asleep or getting enough sleep. This usually means that they are too sleepy in the daytime.

There are many things that influence sleep. Also some substances, called stimulants - coffee is such an example - can cause poor sleep. When people have just eaten something, the body is busy digesting. This can cause poor sleep, too. Worrying and stress can cause poor sleep.

There are many diseases that cause poor sleep. Fever can lead to bad dreams. Poor sleep can be a side effect of some medications.

Sleep disorders directly influence how a person sleeps. Examples of sleep disorders are narcolepsy, sleep apnea and circadian rhythm sleep disorders.

Sleep specialists - doctors specialised in sleeping problems - often suggest better sleep hygiene to people with sleeping problems. Sleep hygiene means things people can try, such as

  • avoid extreme emotion in the hours before sleep
  • trying to get up at the same time every day,
  • sleeping in a cool, quiet and very dark place,
  • avoiding bright light the last hour before bedtime,
  • avoiding a big meal just before bedtime, and
  • getting enough exercise every day.


  1. Macmillan Dictionary for Students Macmillan, Pan Ltd. (1981), page 936. Retrieved 2009-10-1.
  2. Bingham, Roger; Terrence Sejnowski, Jerry Siegel, Mark Eric Dyken, Charles Czeisler, Paul Shaw, Ralph Greenspan, Satchin Panda, Philip Low, Robert Stickgold, Sara Mednick, Allan Pack, Luis de Lecea, David Dinges, Dan Kripke, Giulio Tononi (February 2007). "Waking Up To Sleep" (Several conference videos). The Science Network. Retrieved 2008-01-25.
  3. Guidelines for the Care and Use of Mammals in Neuroscience and Behavioral Research, Institute for Laboratory Animal Research (ILAR), National Research Council, S. pg 121, The National Academies Press 2003, ISBN 978-0-309-08903-6
  4. 4.0 4.1 4.2 "Online Etymology Dictionary". Retrieved 29 September 2010.
  5. Alexander Borbély: Das Geheimnis des Schlafs. Deutsche Verlags-Anstalt, Stuttgart 1984. ISBN 3-421-02734-X
  6. ""Let Sleep Work for You" provided by the National Sleep Foundation".
  7. Rhonda Rowland (2002-02-15). "Experts challenge study linking sleep, life span". Retrieved 2007-04-22.

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