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.
Just as water and food are immensely critical, sleep is crucial to human health and well-being. Humans and animals require sleep in order for their bodies to repose and become invigorated for the next day. Everyday activities, one’s appearance, and how one expresses themselves all rely on this necessity. If one is tired, s/he will not be able to function properly. Being sleep deprived leads to many outcomes such as struggling to remember information, altering one’s mood, health, and a plethora of other continuing effects. Also, the immune system releases compounds known as cytokines which are used to help fight inflammation and infection. If one does not receive enough sleep, then they will not have enough cytokines to protect them from getting sick. The body will also not have time to finalize memory recollection, muscle repair, and release hormones that regulate growth and appetite.
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 or because they are not able to sleep during the day.This is often called a nap. A successful nap should run between 15-30 minutes. Longer naps such as 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 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. "Sleep with someone" can also have a sexual meaning.
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.
Brief stages of sleep[change | change source]
- Stage 1: The lightest sleep of NREM (Non-Rapid Eye Movement) sleep which is the process of falling asleep.
- Stage 2: The first elucidated 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 sleep walking and talking.
- Stage 4: The dreaming stage and 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
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 as well as energy from the brain to the body progressing, the body becoming relaxed, and the eyes dashing back and forth while sleeping. This phase helps prepare one with vitality for performance in 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. 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
People who receive less than 8 hours of sleep a night tend to complain more and feel very fatigued throughout the day. Receiving 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, 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[change | change source]
- Being too full/overeating since the digestive system will not work to digest those foods processed
- Sitting in front of a TV will not help you fall asleep because of the powerful light source being produced
- Drinking too much as it will cause you to use the bathroom multiple times during the night
- Going on a phone or playing a video game makes it hard because the artificial light from the screen simulates the mind and body
- Not having a bedtime routine
- Any type of pain such as back, joint, tooth pain can make you suffer from sleep and you should probably get it checked out
- Having cold feet could also affect it which is why you should wear something to keep warm
- Avoid caffeine at all costs which will result in “all-nighters”
- Stress can make the brain stay active by thinking of all the things on one’s mind
- Snoring can make sleep difficult because you do not have silence and cannot fall back asleep
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 ate. This can cause poor sleep, too. Worrying and stress can cause poor sleep.
The 4 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. While having the urge to move your feet during the night, this 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.
- getting to sleep quick and early
- avoid extreme emotion in the hours before sleep
- trying to get up at the same time every day (sticking to a routine)
- sleeping in a cool, quiet and very dark place with the right mattress, lighting, blanket, pillow and temperature.
- avoiding 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.
- avoiding a big meal just before bedtime
- getting enough exercise every day
- sleeping 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
- Macmillan dictionary for students Macmillan, Pan. 1981, page 936. Retrieved 2009-10-1.
- 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
- Alexander Borbély: Das Geheimnis des Schlafs. Deutsche Verlags-Anstalt, Stuttgart 1984. ISBN 3-421-02734-X
- Low P.S. et al 2008. Mammalian-like features of sleep structure in zebra finches. PNAS 105, 26, 9081-9086.  Discussion section.
- Let sleep work for you. National Sleep Foundation.