Sleep is a state of resting, which happens in animals, including humans. During deep sleep, most of the muscles that animals can otherwise control are not active, but regain the energy for the next time they wake up. Animals during sleep are usually in an unconscious, relaxed state. Like most animals, healthy sleep in humans takes place at night. Asleep describes the condition where animals are in the process of sleeping. During this time, they will not react as quickly (if at all) as they would if they were awake. They can, however, wake up from sleep more easily than from hibernation or coma. All mammals, birds, many reptiles, amphibians and fish have a sleep cycle. In humans, other mammals, and most other animals that were studied, regular sleep is essential for survival.

Sleep is extremely important to human health and well-being. Humans and animals need sleep in order for their bodies to be prepared for the next day. Everyday activities, one’s appearance, and how one expresses oneself all rely on this necessity. If one is tired (from not getting enough sleep), one will not be able to function properly in common activities. Being sleep-deprived leads to struggling to remember information, altering one’s mood, energy, health, focus and a number of other effects. Sleep deprivation (not allowing a person enough sleep) can even be used as torture. Also, the immune system releases compounds known as cytokines which are used to help fight inflammation and infection. If a person does not receive enough sleep, they will not have enough cytokines to protect them from getting sick. The body may not have time to complete memory recollection, muscle repair, and release hormones that regulate growth and appetite.

During daytime, the sun is out and most people are awake. They work, go to school, or complete daily errands and activities. Many people sleep for a short time in the early afternoon for a quick rest—or because they are not able to sleep during the day. This is called a nap. A successful nap should run between 15-30 minutes, and longer naps taking 30-60 minutes will result in feeling dazed and less attentive.

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 close while their owners and/or employees take their siesta.

Word 
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. For example, "sleep" may be used to mean death, so that "putting an animal to sleep" means to kill the animal without pain. "Sleep with someone" can also have a sexual meaning.

What sleep is for 
Generally, the reason for sleep is that the brain needs it. The details are not fully understood, but it is important to get enough sleep for the body and the brain to be healthy and to work properly. In general, 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 up to 15 hours a day (or even longer).

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

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

Brief stages of sleep 
There are four stages during sleep:

 Stage 1: The lightest sleep of NREM (Non-Rapid Eye Movement) sleep which is the process of falling asleep.
 Stage 2: The first stage of NREM sleep; the beginning of falling asleep including regular breathing and heart rates, the body temperature dropping, and becoming disconnected with the environment. 
 Stages 3: Deep NREM sleep which involves delta waves or slow waves. It is difficult to wake one up in the course of this stage as they are in deep sleep. Common disorders that occur during this stage are sleepwalking and talking. 
 Stage 4: The dreaming stage in which brain waves are more vigorous with rapid eye movement. Awakenings are more common in REM (Rapid Eye Movement) sleep as opposed to NREM. 

While humans sleep, REM and NREM are sleep patterns that help with long term memory, remembering information, procedural memory, and creative thinking.

Different types of sleep

REM 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 (rapid eye movement). Most dreams take place in this phase. as the body becomes relaxed and the eyes move while sleeping. This phase helps prepare one for the next day. 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. It is often encountered 90 minutes after falling asleep, and continues to occur every 90 minutes. REM-sleep was first discovered in 195253.

REM sleep is found in mammals and songbirds, but is "poorly established" in reptiles and fish. According to a survey:
"This remarkable similarity of characteristics may have resulted from a convergent evolution in mammals and songbirds".

NREM sleep 
The other category, where this movement of the eyes does not happen, is called NREM-sleep (Non-REM sleep). In general, 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 

People who regularly get less than 8 hours of sleep a night tend to complain more and feel very fatigued throughout the day. Getting the appropriate amount of sleep is extremely important, as it could affect one’s body and increase the chances of serious health problems. For each age group, there are different amounts of sleep that are recommended:

- Toddlers (4 to 12 months): 12 to 16 hours (w/ naps)

- Toddlers (1-2 years): 11 to 14 hours (w/ naps)

- Preschoolers (3-5 years): 10 to 13 hours (w/ naps)

- Grade Schoolers (6 to 12 years): 9 to 12 hours 

- Teenagers(13-18 years): 8 to 10 hours

- Adults (including old age): 7 to 9 hours 

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.

Bad habits 
Poor habits could affect one’s sleep schedule in many ways without taking notice. A few habits that are very common and ruin sleep are:

 Overeating or being too full (since the digestive system will not work to digest those foods processed)
 Sitting in front of a TV (since powerful light source produced by TV can prevent sleep)
 Drinking too much (since it will cause one to use the bathroom multiple times during the night)
 Going on a phone or playing a video game (since the artificial light from the screen simulates the mind and body)
 Not having a bedtime routine 
 Any type of pain such as back, joint and tooth pain (these can making sleeping difficult and should be addressed quickly)
 Having cold feet (it is better to wear something to keep warm in this case)
 Using caffeine (since this can result in “all-nighters”)
 Having stress (since this can keep the brain active at night, thinking of all the things on one’s mind)
 Snoring (since one can be awaken by it)

Sleeping problems 
A good night’s sleep is extremely important for one’s quality of life. 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 eaten. This can cause poor sleep, too. Worrying and stress can also 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.

The four most common sleep disorders are:

Insomnia which consists of difficulties going to sleep at night, having no energy, waking up often in the middle of the night, waking up earlier than planned, and changing mood behaviors.

Sleep apnea which is due to the lack of breathing for several seconds which results in the brain awakening and forcing a respiratory effect to breathe harder. As a result of the multiple occurrences during the night, the body cannot go back to sleep, leading to fatigue.

Restless leg syndrome is a need to move one’s leg while resting. Having the urge to move one's feet during the night may affect the ability to fall and stay asleep.

Narcolepsy, the inability to control the brain’s sleep/wakefulness cycle which leads to daytime sleepiness and falling asleep at unexpected times. 

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

 get to sleep quick and early
 avoid extreme emotion in the hours before sleep
 try to get up at the same time every day (sticking to a routine)
 sleep in a cool, quiet and very dark place with the right mattress, lighting, blanket, pillow and temperature.
 avoid bright light the last hour before bedtime. Eat dinner at least 3 hours before bedtime so the digestive system has time to break it down.
 avoid a big meal just before bedtime
 get enough exercise every day
 sleep in varying positions. However, avoid sleeping on the stomach as it starts to flatten the curve of the spine, which can lead to severe lower back pains

References

Other websites 
 American Academy of Sleep Medicine
 National Sleep Foundation
 World Sleep Foundation 
 National Center on Sleep Disorders Research
 

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