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Sleeping is associated with a state of muscle relaxation and limited perception of environmental stimuli.
Sleep (also called napping) is a state of resting. It happens in most animals, including humans. To sleep is to be unconscious.
During deep sleep, most of the muscles that animals can control are not active, but resting to regain the energy they need for the next time they wake up. When asleep, humans cannot tell what is happening around them. Like most animals, healthy sleep in humans most often happens at night.
When something is asleep, it cannot react like it would if it was awake. However, waking up from sleep is easier than waking up from hibernation or a coma. All mammals and birds, as well as many reptiles, amphibians and fish have a sleep cycle. In humans and many other animals, regular sleep is essential for survival.
It seems that all animals with complex brains have sleep. We know that sleep is extremely important to human health and well-being. Humans and animals need sleep but we do not know exactly why. 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, changing one's mood, energy, health, focus and a number of other effects. This is why people are told to not drive or use big machines when they feel tired. Sleep deprivation (not allowing a person enough sleep) can even be used as torture. Sleep deprivation causes negative effects for the person such as: irritability (annoyed mood), forgetting things or amnesia, and some pains (for example back ache or neck pain) in different areas of the body. Also, the immune system normally releases compounds known as cytokines which are used to help fight inflammation and infection. If a person does not get enough sleep, their body cannot protect them from illness. 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. People work, go to school, or complete daily errands and activities. Some 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 good nap should is recommended to last between 15 and 30 minutes. Naps that are 30–60 minutes can make someone feel dazed, confused, and less focused. It can also make it harder to sleep at night.
In some countries, most notably where the weather is warm, there is a tradition to take a nap early in the afternoon. This tradition is called siesta. It is most common in Spain and Latin America. Some stores and services close while their owners and/or employees take their siesta. It is common because the day gets hottest when the sun is up in the sky and strong. Therefore, people do their activities early in the morning and later in the evening, when it is cooler.
Most of what is said above applies widely in the animal kingdom. However, some primitive animals do not appear to sleep, or more correctly, sleep has never been shown. Some animals sleep as we do, at night, but some sleep during the day and are instead active at night. This is called being nocturnal. Mammals spend much of their early evolution sleeping during the day and being active at night.
Word
The word "sleep" comes from the old Old Germanicverbs 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. This is called a euphemism, because people do not want to say the actual word they mean.
What sleep is for
Sleep helps the brain function. 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 1952–53.
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
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:
Infants (4 to 12 months) require 13 to 15 hours (male) or 15-17 hours (female)
Toddlers (1–2 years): 10 to 12 hours (male) or 13-15 (female)
Preschoolers (3–5 years): 8 to 10 hours (male) or 10-13 hours (female)
Grade Schoolers (6 to 9 years): 7 to 9 hours (male) or 9-12 hours (female)
Teenagers (10–25 years): 5 to 7 hours (male) or 9-11 hours (female)
Adults (including old age): 5 to 7 hours (male) or 8-10 hours (female)
The timing of sleep and the amount of it are both important. Both are different for different people and is based on the circadian rhythm. [source?]
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, and human babies do not develop circadian rhythms before they are 3 – 4 months old.[source?] At the age of 1 year, a baby sleeps for about 14 hours.
A nine-year-old girl should sleep about 9–10 hours per day and teenagers also need that much sleep. Women who sleep less than about 9 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. Some habits that can disrupt 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, phone, or video game (since powerful light source produced by TV and the excitement can prevent sleep)
Drinking too much (since it will cause one to use the bathroom multiple times during the night)
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 (this is a stimulant that makes people feel less sleepy)
Stress (this can keep the brain active at night and can prevent relaxation)
A good night's sleep is extremely important for one's quality of life. However, people may have trouble sometimes 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.
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 -- a lack of breathing for several seconds results in the brain awakening and forcing a breath
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:
