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. 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. We do know for sure, though, that sleep is necessary to most animals for survival. 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 time, most people are awake. They work, go to school, college, 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 common 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. 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).
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.
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.
Different types 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.
REM sleep is found in mammals and songbirds, but is "poorly established" in reptiles and fish. A survey suggests:
- "This remarkable similarity of characteristics may have resulted from a convergent evolution in mammals and songbirds".
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), 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, also need that much sleep. Adults who sleep less than about 8 hours a day perform worse than those who sleep that long.
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 an example – can cause poor sleep. When people have just eaten something, the body is busy digesting what they have ate. This can cause poor sleep, too. Worrying and stress can cause poor sleep.
- 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.
Images for kids
Sleep Facts for Kids. Homework Help - Kiddle Encyclopedia.