show/hide words to know

Ancestor: a person from whom we descended, who lived generations ago.

Deprivation: the state of not being given something or not experiencing something that you normally have or experience.

Foraging: looking for food.

Hypothesis: a possible reason or explanation for an observation.

Inflammation: a response by damaged or infected cells to attract immune cells.

Metabolism: what living things do to stay alive. This includes eating, drinking, breathing, and getting rid of wastes... more

NREM sleep: the non-rapid eye movement phase of sleep, during which it can be challenging to wake up.

REM sleep: the rapid eye movement phase of sleep, during which we wake easily....more

Why Do We Sleep?

People spend nearly 1/3 of their lives sleeping. Oddly, we still are not completely sure why. Here, we’ll explore some of hypotheses for why we sleep. It’s important to keep in mind that all of these hypotheses may be partially true—there probably isn’t just one reason as to why we sleep.

Improved Health


Getting enough sleep is important for a healthy lifestyle. Long-term lack of sleep can lead to increased stress and many other problems. Image by Sander van der Wel.

If you’ve ever missed a night of sleep, you may feel sluggish, hungry, and maybe weirdly hot or cold on the next day. If you deprive yourself of sleep for days and days, you may start getting sick a lot and gain weight. Sleep regulates many internal processes in your body. Long-term sleep deprivation can alter metabolism and increase stress. These changes can lead to increased body temperature, weight gain, high blood pressure, strong inflammation, and a reduced ability to fight off infections.

Sleep Helps Learning and Memory

Your teachers and parents have probably advised you to get a good night’s sleep before taking exams. This advice is based on the popular belief that sleep helps with memory retention. Research has provided some evidence to back up this advice. A good night’s sleep includes REM sleep, which helps with remembering procedures, foreign languages, logic, and emotions. However, REM sleep is not necessary for processing long-term memories of facts, experiences, and concepts. Therefore, while sleep will help you ace that exam, it will not guarantee it.

Energy Conservation and Safety

Foraging for food

Daylight hours are important for gathering food and for some hunting, which may partly explain why we sleep at night. Image by JackyR.

Do you ever wonder why we sleep at night as opposed to during the day? One explanation is that our ancestors needed daylight to do activities such as hunting and foraging. Furthermore, our ancestors also needed to avoid predators that roamed around at night. By sleeping, our ancestors were able to conserve energy while they weren’t able to search for food, and were able to escape the detection of predators by staying still and keeping quiet.

Quality, Not Quantity

If sleep is so great for our health and mind, then why didn’t humans evolve to sleep longer? To answer this question, scientists compared the benefits of staying awake, such as finding food and interacting with each other, with the benefits of sleeping. Overall, scientists found that only daily habits appeared to have a significant impact on sleep duration. This means that, in general, we will sacrifice increased amounts of sleep (and the associated positive effects on the body) for more time to engage in daily activities, like looking for a mate, finding food, or other positive behaviors. These other activities appear to have allowed us to learn and do things that outweigh the negative effects of sleep deprivation on the body.

Additional images via Wikimedia Commons. Girls chatting by Rod Waddington.

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Bibliographic details:

  • Article: Why Do We Sleep?
  • Author(s): Sisi Gao, Patrick McGurrin
  • Publisher: Arizona State University School of Life Sciences Ask A Biologist
  • Site name: ASU - Ask A Biologist
  • Date published: June 21, 2017
  • Date accessed: June 28, 2022
  • Link:

APA Style

Sisi Gao, Patrick McGurrin. (2017, June 21). Why Do We Sleep?. ASU - Ask A Biologist. Retrieved June 28, 2022 from

American Psychological Association. For more info, see

Chicago Manual of Style

Sisi Gao, Patrick McGurrin. "Why Do We Sleep?". ASU - Ask A Biologist. 21 June, 2017.

MLA 2017 Style

Sisi Gao, Patrick McGurrin. "Why Do We Sleep?". ASU - Ask A Biologist. 21 Jun 2017. ASU - Ask A Biologist, Web. 28 Jun 2022.

Modern Language Association, 7th Ed. For more info, see

Humans will often trade off sleeping hours with the chance to socialize and form stronger community bonds.

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