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Why Do We Need Sleep: Science-Backed Facts

Moritz Jaax

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Why do we need sleep? While we sleep, parts of our bodies continue to work at full speed. Did you know that one-third of people in the United States have reported that they get less than the recommended seven to nine hours of sleep per night?[17, 18] A lack of sleep has been associated with health conditions, such as type 2 diabetes, cardiovascular disease, obesity, and depression. It is therefore vital that we each get enough sleep to keep our bodies fighting fit.

From an evolutionary point of view, sleeping is actually a bad idea. You are essentially rendered defenseless for hours without being able to fend for yourself. And you can’t hunt for food during this time either. Nevertheless, almost all animals and humans need to sleep every night to stay healthy, indicating the importance of sleep – that is, the functions sleep carries out when we are asleep.

Much of what happens when we rest is still a mystery, even to researchers. Nevertheless, we go into greater depth about the function of sleep and what happens in our bodies and brains while asleep –  from consolidating memories, producing immune cells, and boosting cell regeneration. Discover about our circadian rhythm and how many hours of sleep you really need for your age.

Why Do We Need Sleep?

It’s not for nothing that we spend around a third of our lives asleep. Sleep is vital – and if you sleep too little or too poorly, it greatly affects your well-being and performance and increases the risk of various diseases.

Scientists do not yet understand all the effects and function of sleep. What is crystal clear, however, is that sleep is essential. We need it to regenerate and for our brain to process information and thoughts.

Our body therefore has processes in place to ensure we get tired and want to sleep. This process is controlled primarily by two bodily processes known as our circadian rhythm and sleep homeostasis.

man pressing snooze on his alarm clock

What Is the Function of Sleep?

The vital functions of sleep include:[7–9]

  • Development: The brain, in particular, develops significantly at night. This could be one of the reasons why newborns and children sleep so much.
  • Regeneration and wound-healing: Our bodies release growth hormones and produce new cells, which helps the body regenerate. Researchers have been able to observe in rats that wounds heal worse when they suffer from sleep deprivation.
  • Boosting the immune system: Important immune defense processes take place during sleep. When we sleep, our bodies produce immune cells and its immunological memory, with which the body recognizes pathogens and foreign substances. Studies suggest that too little sleep increases the risk of infections.
  • Saving energy: Studies show that people need significantly less energy when they sleep than when they simply rest while awake. So without sleep, we would use up significantly more energy.

When you are ill, you often feel the need to sleep more. Various inflammatory factors in the body most likely play a role here, making you sleepy and thus indirectly encouraging you to take it easy. Researchers are currently exploring the many links between our immune system and sleep.[9]

How Do Animals Sleep?

Sleep is something that exists in various forms throughout the animal kingdom. Birds and mammals, in particular, have very similar sleeping habits to humans.

While there is debate among researchers as to whether very specific species of frogs and birds actually sleep, so far, scientists have not been able to conclusively prove that any species does not engage in at least some form of nocturnal or daytime rest.[3]

Marine mammals such as whales, dolphins, and seals are an exception. They sleep with only one-half of their brain while the other half remains awake. This allows them to emerge from the water regularly to catch their breath despite being asleep.[3]

Did you know that the bigger mammals are, the less sleep they usually need? Bats sleep 18 to 20 hours a day, while cats and mice sleep 12 to 13 hours. Many very large animals, such as giraffes, elephants, and horses, sometimes sleep only three to four hours or even less in the wild.[4]

What Is the Circadian Rhythm?

The circadian rhythm, also known as the circadian clock, is our internal clockwork that changes our bodily functions over the course of day and night. In the late evening, our body temperature and blood pressure drop, and at the same time, our brain secretes the hormone melatonin, which makes you tired and helps you fall asleep.

Melatonin production depends mainly on how much light we expose our eyes to. However, people are wired differently and have a different circadian rhythm – some people get tired earlier in the evening, others later. Artificial light and especially screen time can disrupt our circadian rhythm and cause people to go to bed later and later.

You can find several sleep supplements these days that boost your melatonin levels, so that you can fall asleep more easily. These supplements are usually recommended for those regularly struggling to fall asleep quickly – it is believed that these people have low melatonin levels in the evening.

When sleep is lost, this loss is compensated by extending subsequent sleep. This homeostatic aspect is thought to be one of the main regulatory processes in sleep and seems to be universal, as it is found in many different phyla of the animal kingdom

What Is Sleep Homeostasis?

Sleep homeostasis refers to the regulatory aspect of our body when it lacks sleep, for example. When we are deprived of sleep, this loss is compensated by sleeping for longer the next time we sleep – similarly to when we are deprived of food and drink, our need to eat and drink increases and must eventually be satisfied. This regulatory process is also found in most animals. 

This happens, among other things, through the messenger substance adenosine. The longer you are awake, the more adenosine accumulates in the body, which, in turn, increases our need for sleep. Caffeine can block the receptors to which adenosine docks – presumably, this is how coffee, black tea, and caffeinated soda keep us awake.[1, 2]

What Are the 4 Phases of Sleep?

Sleep can be divided into four phases that are repeated again and again over the course of the night: falling asleep, light sleep, deep sleep, and REM (rapid eye movement). One cycle of these phases lasts about 90 minutes, whereby the individual phases shifts over the course of the night. When you fall asleep, you are initially in a deep sleep for longer, but toward the end of the night, you spend more time in the REM phase.

These phases are significantly different. For example, your brain is highly active during the REM phase. In this phase, you have dreams that you are most likely to remember. In the deep-sleep phase, on the other hand, your body fully rests, you hardly dream, and are difficult to wake up.

Function of Sleep: What Happens to Our Bodies When We Sleep?

One of the most important functions of sleep is processing information: Memories are stored, what we have learned throughout the day is processed, and less relevant information is erased. You could also say that our brains clean up all of this information. It cuts certain connections between synapses, while others remain and are consolidated. Other nerve connections are also newly formed.

By doing this, our brains allow us to distinguish between relevant information and superfluous information that we have been hit with throughout the day – this frees up memory, so to speak, that we can fill up again the next day.

This is also the reason why we learn things more effectively if we sleep well afterward. Our brain only makes necessary connections between information during the various sleep phases. So pulling an all-nighter before an exam or an important presentation is absolutely not a good idea. Your brain will be able to process what you learn much better if you get an adequate amount of sleep.[10, 11]

Why do we dream? Scientists still don’t understand the exact function of dreams. Presumably, they help us to process emotions that we had to deal with during the day. What is clear is that everyone dreams for around two hours every night – even if many of us rarely remember our dreams.[1]

How Many Hours of Sleep Do I Need?

When we talk about adults, the average amount of time recommended by the US-American Academy of Sleep Medicine is around seven to nine hours of sleep per night – studies have shown that this amount of sleep leaves adults feeling refreshed and awake. How much sleep we need varies, however, from person to person.

There are people who naturally get by on five hours of sleep per night; however, there are not many of these people. Often, too little sleep tends to affect people without them noticing because they have become accustomed to a constant lack of sleep. Nevertheless, this can have harmful long-term consequences.

When should you go to bed? The important thing is to listen to your circadian rhythm and, if possible, go to bed at the times when you feel tired. As a night owl, you won’t be doing yourself any favors if you try to go to bed at 10 p.m. However, you should aim to go to bed at the same time every day, even on weekends. This helps your body to adjust to a fixed rhythm.

How Much Sleep Do I Need for My Age?

Very young children in particular still sleep significantly more than adults. As time goes by, we need less and less sleep – until the sleep requirement in adulthood settles at an average of seven to nine hours.

This table reveals the recommendations of US sleep experts for the respective age groups:[14]


Recommended Sleep Duration

0–3 months

14–17 hours

4–11 months

12–15 hours

1–2 years

11–14 hours

3–5 years

10–13 hours

6–13 years

9–11 hours

14–17 years

8–10 hours

18–64 years

7–9 hours

65+ years

7–8 hours

In addition to this recommended sleep duration, the US-American Academy of Sleep also gives ranges for adequate hours of sleep. For adults between 26 and 64, for example, this range is six to ten hours. So anything within this range is unproblematic if you feel comfortable and well-rested. If you consistently sleep more than ten or less than six hours, it may be worth clarifying your sleeping regime with a doctor or therapist.

What Side Effects Are Caused by Too Much Sleep or Too Little Sleep?

If you consistently sleep more than ten hours or less than six hours, you are outside the recommendations of sleep medicine. If this applies to you, you should discuss your sleep with a doctor or therapist or to try to improve your sleep regime yourself.

What Happens When You Lack Sleep?

A chronic lack of sleep makes you sleepy and fatigued during the day; it also reduces physical and mental performance. Among other things, people who sleep too little are able to concentrate less, they are more forgetful, and they have slower reflexes.

Long-term sleep deprivation also increases the risk of many diseases, such as depression, anxiety disorders, cardiovascular diseases, diabetes, and obesity.[12]

What Is the Effect of Too Much Sleep?

Should there also be a maximum threshold of sleep set by experts in order to promote our health and well-being? Whether sleeping more than nine hours a night could cause health problems is not yet clear, according to medical experts.

There are some major studies that show that people who sleep nine hours or more a night have a lower life expectancy on average. However, some scientists think it is possible that this is a purely statistical correlation. People who sleep a lot often live with other conditions that can negatively affect their health, such as low socioeconomic status or depression.[16]

Needing sleep critically can also be a sign that you are sleeping poorly or inefficiently, or that you regularly need to catch up on sleep from other nights.[12, 14]

Napping: Is It Possible to Catch Up on Sleep?

Your need for a good night’s sleep is all the greater when we have had a few nights of poor sleep or too little sleep. In fact, you can catch up on sleep and then be more alert and focussed again.

Researchers have investigated this in a laboratory study. The test persons were only allowed to sleep for four hours at a time for a few days. In the nights that followed, they slept longer on average than they normally would. However, it took most of them several nights of catching up before they regained their full mental capacity.[13]

Do Moon Phases Affect Your Sleep?

woman lying in bed trying to sleep

Many people claim that they sleep worse when there is a full moon. Although this may sound bizarre, the scientific link between a full moon and sleep quality is actually a controversial topic in sleep research!

Some scientists believe that sleeping problems during a full moon could be a relic of human evolution. Studies with people who still live as traditional hunter-gatherers and largely without electric light reveal that these people go to bed later on nights with a full moon because there is more light.

Researchers are working on a theory based on the behavior of our ancestors. The bright moonlight was the reason our ancestors stayed up later on nights where there was a full moon. Today, we can avoid moonlight by closing our curtains or blinds, but our bodies are still able to notice that there is a full moon, for example, because of the greater gravitational force of the moon. Our bodies may still associate this with the brighter, more active full-moon nights our ancestors experienced – and enable us to sleep less.[5, 6]

Why Do We Need Sleep – at a Glance

Why Do We Need Sleep?

Our circadian rhythm – that is, our inner clock – controls when we are awake or asleep. In the evening, our brains release the hormone melatonin, which makes us feel tired and fall asleep.

Secondly, our need for sleep builds up in the body the longer we are awake – this is what we call sleep homeostasis. Among other things, the messenger substance adenosine accumulates in our bodies during the day and is only broken down again during sleep.

Why Is Sleep So Important?

During sleep, important processes take place in the body that we need to survive.

Among other things, we store memories and process what we have learned during the day – so, the brain tidies up the information we have received during the day at night.

Immune cells are produced during our sleep and immunological memory is formed. This is probably the reason why people who sleep too little often become ill.

In addition, regeneration and wound healing take place more intensively at night and help injuries heal and muscles grow.

How Long Should I Sleep?

Our sleep requirements vary from person to person. On average, adults need between seven and nine hours of sleep – although, it is not uncommon for some to get by on six hours and others to need up to ten.

Babies, children, and adolescents sleep more than adults, and 14 to 17 hours of sleep per day is still recommended for newborns.


[1] Brain Basics: Understanding Sleep | National Institute of Neurological Disorders and Stroke, available at, accessed on May 11, 2021.

[2] Cajochen C. Schlafregulation, Somnologie - Schlafforschung Schlafmed., vol. 13(2), pp. 64–71, June 2009, doi: 10.1007/s11818-009-0423-7.

[3] Cirelli C., Tononi G. Is Sleep Essential? PLoS Biol., vol. 6(8), August 2008, doi: 10.1371/journal.pbio.0060216.

[4] Gravett N. et al., Inactivity/sleep in two wild free-roaming African elephant matriarchs – Does large body size make elephants the shortest mammalian sleepers? PLOS ONE, vol. 12(3), S. e0171903, March 2017, doi: 10.1371/journal.pone.0171903.

[5] Casiraghi L. et al. Moonstruck sleep: Synchronization of human sleep with the moon cycle under field conditions, Sci. Adv., vol. 7(5), S. eabe0465, January 2021, doi: 10.1126/sciadv.abe0465.

[6] Cajochen C., Altanay-Ekici S., Münch M., Frey S., Knoblauch V., Wirz-Justice A. Evidence that the Lunar Cycle Influences Human Sleep, Curr. Biol., vol. 23(15), pp. 1485–1488, August 2013, doi: 10.1016/j.cub.2013.06.029.

[7] Jung C. M., Melanson E. L., Frydendall E. J., Perreault L., Eckel R. H., Wright K. P. Energy expenditure during sleep, sleep deprivation and sleep following sleep deprivation in adult humans, J. Physiol., vol. 589(Pt 1), pp. 235–244, January 2011, doi: 10.1113/jphysiol.2010.197517.

[8] GÜMÜSTEKIN K., et al. Effects of Sleep Deprivation, Nicotine, and Selenium on Wound Healing in Rats, Int. J. Neurosci., vol. 114(11), pp. 1433–1442, January 2004, doi: 10.1080/00207450490509168.

[9] Besedovsky L., Lange T., Haack M. The Sleep-Immune Crosstalk in Health and Disease, Physiol. Rev., vol. 99(3), pp. 1325–1380, July 2019, doi: 10.1152/physrev.00010.2018.

[10] Yang G., Lai C. S. W., Cichon J., Ma L., Li W., Gan W.-B. Sleep promotes branch-specific formation of dendritic spines after learning, Science, vol. 344(6188), pp. 1173–1178, June 2014, doi: 10.1126/science.1249098.

[11] Rasch B., Born J. About Sleep’s Role in Memory, Physiol. Rev., vol. 93(2), pp. 681–766, April 2013, doi: 10.1152/physrev.00032.2012.

[12] Chaput J.-P., Dutil C., Sampasa-Kanyinga H. Sleeping hours: what is the ideal number and how does age impact this? Nat. Sci. Sleep, vol. 10, pp. 421–430, November 2018, doi: 10.2147/NSS.S163071.

[13] Banks S., Van Dongen H. P. A., Maislin G., Dinges D. F., Neurobehavioral Dynamics Following Chronic Sleep Restriction: Dose-Response Effects of One Night for Recovery, Sleep, vol. 33(8), pp. 1013–1026, August 2010.

[14] Watson Nathaniel F. et al. Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement of the American Academy of Sleep Medicine and Sleep Research Society, J. Clin. Sleep Med., vol. 11(06), pp. 591–592, doi: 10.5664/jcsm.4758.

[15] Techniker Krankenkasse, Schlaf gut, Deutschland - TK-Schlafstudie 2017, 2017.

[16] Patel S. R., Malhotra A., Gottlieb D. J., White D. P., Hu F. B. Correlates of Long Sleep Duration, Sleep, vol. 29(7), pp. 881–889, July 2006, doi: 10.1093/sleep/29.7.881. 

[17] “Sleep and Sleep Disorders, Centers for Disease Control and Prevention, available at, accessed on November 15, 2021.

[18] “How Much Sleep Do I Need?” Centers for Disease Control and Prevention, available at, accessed on November 15, 2021.

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