Why Do We Sleep? The Evolution of Rest According to Science

Medically reviewed by
 Dr. Nayantara Santhi

Dr. Nayantara Santhi

Dr. Nayantara Santhi holds an academic position at Northumbria University. After completing her Ph.D. at Northeastern University (Boston, MA), she joined the Division of Sleep Medicine at Harvard Medical School as a post-doctoral fellow to research how sleep and circadian rhythmicity influence our cognitive functioning.

Read more

By Rosie Osmun Certified Sleep Coach

You’re designed to drift off to dreamland several hours each day, but have you ever wondered why you sleep? Most life forms require rest to live. That we need sleep…

Last Updated On November 13th, 2022
Why Do We Sleep? The Evolution of Rest According to Science

You’re designed to drift off to dreamland several hours each day, but have you ever wondered why you sleep?

Most life forms require rest to live. That we need sleep to thrive and survive is undisputed, but researchers are just beginning to understand how sleep works, why we need sleep, and what exactly happens during rest.

Black Friday Sale

Save $450 On Any Mattress

Plus free shipping

Get $450 OFF Mattresses

To many people, the concept of sleep appears perplexing. Why would we be designed or have evolved to spend significant portions of the day unconscious and vulnerable? What advantage does this serve, and what happens during sleep that can’t happen while we are awake?

In this article, we’ll take a look at current scientific opinions on how sleep evolved, what happens during rest, and what purpose it serves.

How Did Sleep Evolve?

Circadian rhythms are biological rhythms that cycle through every 24 hours, and are present in all plants and animals. Circadian rhythms are governed by both internal and external cues, and among other things, this “internal clock” governs periods of rest and alertness.

All of today’s mammals and birds engage in sleep, though different species have different rest needs. Many fish and insects that have been researched show sleep-like states as well, and reptiles also show evidence of resting states.

The pervasiveness of sleep in both vertebrates and invertebrates suggest that sleep has been around for hundreds of millions of years. Scientists are still trying to find ways to investigate whether simpler organisms like bacteria require rest in order to better understand how this process has developed over time.

There is no singular scientific opinion regarding why organisms, including humans, developed the need to sleep because there is still much yet to be discovered.

Evolution of Sleep in Humans

Though much of what is known about the development of sleep is still theoretical, it is believed that sleep mainly serves to restore and conserve energy. But, modern research has found many aspects of health are intimately connected with sleep, not just energy, adding even more complexity and mystery to the equation.

Changes in Sleep Over Time

While our need to sleep is shared with our ancestors and genetically-driven, our environments have likely contributed to changes in how people sleep over time.

Prior to the Industrial Age, there are ancient literature and historical accounts of people sleeping in two phases, rather than for 7-9 hours in one go as many people do now. Termed segmented sleep, people described sleeping shortly after sundown for several hours, waking during the night to socialize, eat or engage in other activities, and then returning for a second round of sleep until daylight.

In the 1800s, as people began to congregate in urban centers, engage in the industrial revolution and enjoy widespread gas lighting and later electricity, sleep habits began to change. Excess sleep was seen as lazy, and well-lit nights extended people’s activities well past sundown. Recent studies have also shown that electronic lights can indeed affect melatonin levels and sleep.

Today, the U.S. Centers for Disease Control and Prevention Verified Source Centers for Disease Control and Prevention (CDC) The United States’ health protection agency that defends against dangers to health and safety. View source say that we are in the midst of a national sleep epidemic, and research suggests that a significant proportion of the population does not get enough rest, holding significant implications for health, public safety and economic productivity.

Why Do We Sleep?

Scientists have been studying sleep since at least 500 B.C., and likely well before that. However, despite amazing advances in the past century, much is still unknown about exactly why people and animals require sleep or how complex biological processes work.

Studies have shown that both mental and physical health depend on getting adequate rest. Sleep has clear genetic links in both animals and humans, with studies establishing genes related to sleep regulation and the biological functions that manage sleep and wakefulness.

While we now know that sleep is essential and that there are many functions dependent on or affected by sleep, there still is not a clear consensus as to why we evolved to need sleep. Some popular theories outlined by Harvard’s Healthy Sleep website include:

  • Inactivity Theory – Organisms evolved to sleep at night to protect themselves from danger.
  • Energy Conservation Theory – Sleep reduces an organism’s energy needs at night when it’s not efficient to search for food.
  • Restorative Theories – Sleep allows an organism to repair and rejuvenate after the day’s activities, improving immunity, healing damage, promoting growth and removing waste.
  • Brain Plasticity Theory – Sleep and the REM (rapid eye movement) cycle allows the brain to develop new connections, learn and process memories. Both REM and deep sleep (also known as non-REM sleep) are especially important for brain health. The rapid eye movements that occur during this phase are an indication of dreaming. Certain brain waves, called theta waves, are also produced during these critical phases of sleep.

Other researchers posit that sleep serves multiple complex purposes, encompassing several or all of the above theories.

Biological Sleep-Wake Cycles

Even though light and lifestyle changes may mean we snooze differently than ancestors, the basic functions of sleep likely remain unchanged. There is still a lot to be learned about the complex biological process behind sleep Verified Source Harvard Health Blog run by Harvard Medical School offering in-depth guides to better health and articles on medical breakthroughs. View source and consciousness, but research has gleaned some important information.

In humans, as well as most mammals, sleep is regulated by an internal clock, that is in turn regulated by complex internal and external feedback. Our circadian rhythms run in a cycle lasting about 24.2 hours, on average.

Circadian rhythms do vary between people, with genetics showing clear evidence of early and late preferences. Age, environment and your routine can also affect your circadian clock and sleep cycles.

Internally, your circadian clock regulates body temperature and feelings of wakefulness and sleepiness. When your core body temperature drops (usually occurs in the very late evening and into the night.), you feel drowsiness.

Your pineal gland releases melatonin to encourage sleep and shuts down areas of the brain that stimulate arousal. The build-up of adenosine and depletion of glycogen in your brain are also associated with the drive to sleep, though there are other neurotransmitters like histamines and orexin that can increase arousal and delay sleep.

Externally, light plays a significant role. Light is detected visually and transmitted by optic nerve cells to the suprachiasmatic nucleus (SCN), which resides in your brain’s hypothalamus and is believed to be the functional circadian clock that regulates the timing of your sleep-wake cycle.

The detection of light suppresses melatonin, the hormone responsible for inducing drowsiness, and causes the SCN to take other actions to keep you alert. Consumption of caffeine can suppress adenosine, delaying tiredness, while noises, temperature, when and what you eat, and even traveling all can affect circadian rhythms as well.

What happens when we sleep?

Your brain does not shut down during sleep; in fact, it remains quite active, cycling through several different stages of rest. Blood vessels dilate to allow for efficient removal of waste products that build up as you fire off neurons throughout the day.

It’s believed memories of events and things you learned are processed and consolidated while you rest, and neurotransmitters and some hormones are also regulated while you’re at rest.

During sleep, cell repair and growth accelerates. Skin and organs work to heal damage from stress and oxidation, while growth hormones are released.

The repairing and rejuvenating effects are likely behind the concept of “beauty sleep,” and one study found Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source that people were rated as looking less attractive and less healthy following sleep deprivation than when they were well-rested.

Messy and empty bed with blanket
Photo by Bigstockphoto

What happens when we don’t sleep?

Science has also learned much about what goes on when we sleep by studying the effects of sleep deprivation in animals and people. Research has found that after even temporary sleep deprivation, changes take place at the genetic level, impacting several functions from immunity to stress to carbohydrate metabolism and several other processes.

Over time, a lack of sleep can place a person at higher risk for heart problems, diabetes, weight gain and obesity, and Alzheimer’s disease. In addition to physical effects, a lack of rest (and particularly REM sleep) has also been shown Verified Source Harvard Health Blog run by Harvard Medical School offering in-depth guides to better health and articles on medical breakthroughs. View source to impair decision making, reaction times, mood, motivation, learning and may also affect memory.

“Dreaming is a characteristic aspect of REM sleep,” notes Dr. Nayantara Santhi. “Dreams can last between 5 and 45 minutes When in REM sleep your brain is as active as it is when you are awake. But body paralysis prevents sleepers from acting out vivid images and sounds in their dreams.”

While there are obvious ethical limits to sleep deprivation studies in people, in a 1983 study of rats, Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source animals deprived of sleep died within a few weeks, and a condition that prevents some people from sleeping (Fatal Familial Insomnia) also results in death, demonstrating that sleep truly is a necessary function to sustain life.


Why do humans sleep?

Humans need sleep for our bodies to restore and repair themselves for each new day. Without sleep, we lose concentration easily, have poor reaction times, and are unable to complete basic tasks without feeling drowsy or unfocused.

Going without enough sleep long-term can lead to more serious issues, such as heart disease, depression, anxiety, and diabetes. Conversely, sleeping too much can lead to similar health problems too. Don’t forget that infants need more sleep than adults because they are developing rapidly.

What are the benefits of sleeping?

7-9 hours of sleep per night (for adults) has a wealth of benefits, including a healthy immune system, preventing weight gain, a healthier heart, an even temperament and mood, less risk of depression and anxiety, better productivity, improved memory, and improved physical performance.

What happens when we are sleeping?

During sleep, your body cycles through three stages of non-REM or NREM sleep and a stage of REM sleep. While sleeping, your body moves information to long-term memory and repairs muscles, among other things. Completing all stages of sleep is crucial for overall health. While you’re asleep, your heart rate slows and your body temperature drops, but your brain is still active in a way that’s distinct from wakefulness.

What does our brain do when we sleep?

Though you usually don’t remember any of it, your brain is busy during sleep! While asleep, your body removes toxins from the brain that can lead to brain disorders later in life, such as Parkinson’s or Alzheimer’s. If your time asleep decreases, so will the frequency of these critical processes.

Which age group sleeps the longest?

Naturally, newborn babies sleep the most out of any age group, sleeping 12 to 16 hours every day. As the baby grows up the amount of sleep they need will gradually drop off. Until they’re a full adult, who sleep about 7 to 9 hours on average.

Science Is Still Learning About Rest

Given the findings of modern research, sleep clearly has critical effects on the body at the cellular level beyond simply allowing the body and mind to rest. During sleep, your body and brain undertake many activities, from clearing out waste products to accelerating healing.

Though we don’t know exactly why people evolved to require daily rest, it is clearly needed to maintain health and well-being. Some areas of research are seeking ways to make sleep more efficient, or to prevent the effects of sleep deprivation from impacting the body.

The more we know about sleep, the better equipped we are to understand how to get optimal rest, how to treat sleep disorders, how the brain works, and even how to potentially reduce sleep needs or make sleep more efficient in the future.

Given the rapid advances in our understanding of sleep just over the past few decades, it will be exciting to see what science has in store in the near future and how this could change our relationship with rest.

What do you find most intriguing or perplexing about the concept of sleep? Does understanding the science of rest make sleep more valuable?

P.S. If you liked this post, you might also enjoy Dream Hacking: Understanding Everyday Behaviors That Impact Your Dreams or The DIY Guide to Reduce Fatigue & Excessive Sleepiness.

About the author

Rosie Osmun regularly contributes to the Amerisleep blog writing about topics including, reducing back pain while sleeping, the best dinners for better sleep, and improving productivity to make the most of your mornings. She finds the science of sleep fascinating and loves researching and writing about beds. Rosie is also passionate about traveling, languages, and history.

View all posts

Discover the ultimate sleep system

Choose your mattress

Shop top-rated mattresses with proven sleep-boosting materials.

Get a pillow

We have the perfect pillow to pair with your mattress.

Browse Pillows

Pick out bedding

Bring out the best in your mattress with our soft and breathable bedding.

Browse Bedding