Why We Sleep (Plus What Happens In the Brain and Body When We Do)

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As a human being, you’ll spend anywhere from one-quarter to one-third of your life asleep. You might try to beat the odds by sleeping less and staying awake more, but eventually, you will sleep. Because if you don’t, you’ll die. That may sound pretty dire, but it’s also the truth: Sleep is essential for human life. In spite of this necessity, there’s still a lot of uncertainty around exactly why we sleep.

You’d think it would be obvious why something as essential as drinking water and eating food would be a necessary part of human existence. But in fact, researchers are still working to understand the specific biological purpose that is served when human bodies fall asleep on a regular basis.

Of course, sleep scientists have their theories. Here’s what science says so far about why we sleepplus how much sleep humans need, what happens in the body when we sleep, what happens when we don’t sleep enough, and how quality sleep is essential for optimal wellbeing.

Note: The content on Sleepopolis is meant to be informative in nature, but shouldn’t be taken as medical advice or take the place of medical advice from a trained professional. If you feel you may be suffering from any sleep disorder or medical condition, please see a healthcare provider.

Different Theories About Why We Sleep

As humans, we all possess a sleep drive, or the urge to sleep. No doubt you’ve experienced this in your own life: If you go too long without sleep, you start to feel sleepy. That feeling is your sleep drive in action.

Scientists know that most organisms have a sleep drive. (Even tiny worms with just a few hundred neurons demonstrate sleep-like activity!) What they’re less clear about is exactly why we have a sleep driveor why we sleepin the first place.

Over the years, a number of possible explanations have emerged. According to researchers at Harvard University’s Division of Sleep Medicine, several theories may help explain why we sleep on a daily basis:

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Why We Sleep

Inactivity Theory

Also known as the “adaptive” or “evolutionary” theory, this is one of the longest-standing theories about sleep. The theory suggests that animals (including humans) evolved to be inactive at night as a means of staying safe.

The idea is that animals learned to be still and quiet during a time when they might otherwise be vulnerable to predators or accidents (such as falling off a cliff because they didn’t see it in the dark) if they were out moving around. Those animals who were inactive at night were more likely to survive than animals that remained active at night, and were thus favored by natural selection.

One limitation of this theory is that it doesn’t account for unconsciousness. Counter-arguments point out that, presumably, it would be safer for a creature to be both inactive and alert rather than inactive and unconscious. Thus, this theory may not tell the full story when it comes to why we sleep.

Energy Conservation Theory

In order to survive, animals (including humans) need energy stores. The more energy animals burn, they more fuel they need to take in. Thus, from an evolutionary perspective, it’s in animals’ best interests to conserve energy as much as possible so they aren’t needing to constantly search for food. That’s the basic idea behind the Energy Conservation Theory of sleep.

This theory purports that sleeping for part of the day and/or night reduces the amount of energy an animal expends and decreases their energy demands. This helps free them from needing to search for food when it’s not efficient or safe to do so (say, in the dark of night).

This theory makes sense when you consider that we use significantly less energy when we sleep than we do when we’re awake. Our bodies don’t need to work as hard to sustain themselves, and they require fewer calories for self-regulation than they do when we’re awake. This information helps some scientists feel confident that the Energy Conservation Theory at least partly explains why we sleep. Some researchers also believe this theory and the Inactivity Theory are related.

Restorative Theory

Yet another theory that may help explain why we sleep is the idea that sleep helps our bodies “restore” elements that may be drained or impaired while we’re awake. For example, during sleep, our bodies get to work growing muscles, repairing tissues, synthesizing proteins, and so on. They also complete various hormonal and cognitive cycles.

This theory is supported by research that suggests sleep is essential for the survival of the human body. For example, studies have found that depriving animals of sleep causes their immune system function to shut down and can lead to death in a matter of weeks.

Brain Plasticity Theory

The Brain Plasticity Theory is one of the more recent theories about the value of sleep, but it’s already gained a lot of traction within the scientific community. Research has found that sleep is associated with changes in “brain plasticity,” or the way the brain is structured and organized. In other words, sleep may be essential for cognitive development and function.

This theory is supported by studies that suggest sleep is essential for brain development in infants and young children—which may help explain why these age groups have higher sleep needs than adults with fully-formed brains. That’s not to say adults aren’t also impacted by the Brain Plasticity Theory. Research into the cognitive impacts of sleep deprivation in adults suggests sleep is essential for healthy brains at every stage of life.

This concept is further supported by studies that suggest even though you’re not conscious during sleep (unless you’re having a lucid dream), a lot is happening in your mind. There’s evidence that several areas of the brain play a big role in sleep. These include the hypothalamus, the suprachiasmatic nucleus (SCN), the brain stem, the thalamus, the cerebral cortex, the pineal gland, the basal forebrain, the midbrain, and the amygdala.

Each of these brain areas affects sleep and is affected by sleep in different ways. There’s also evidence to suggest that sleep impacts the way neurons from these different brain regions communicate with each other. The strong connection between the brain and sleep lends credence to the Brain Plasticity Theory.

Reproductive “Fitness” Theory

Another new theory on the block is the idea that sleep somehow plays a role in sustaining humans’ reproductive “fitness”—or their capacity to reproduce and sustain a strong family tree. This idea is based on research that suggests humans who sleep well and on a regular basis are more likely to demonstrate strong reproductive function.

While each of these theories has its own merits, many sleep experts suspect that a combination of these ideas (rather than one theory in isolation) may ultimately hold the answer to why we sleep.

How Much Sleep Do Humans Need?

If you’ve read anything about sleep, then you’ve probably come across the idea that everyone should sleep for seven to eight hours per night. So it might seem like the question of how much sleep humans need is an open-and-shut case.

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How Much Sleep Do Humans Need?

In reality, the question is slightly more complicated. Different people require different amounts of sleep depending on their life stage, genetics, circadian rhythms, and other factors. For instance, recall from the previous section that infants, young children, and teens need to sleep far more than adults.

That being said, for the most part, sleep experts agree that sleeping for seven to eight hours a night is ideal for most adults. Sleep less than that on a consistent basis, and you risk incurring all the negative consequences of sleep deprivation. (More on that below.)

While most people are aware that sleep deprivation isn’t good, fewer people realize that too much sleep can also be potentially damaging.

One large meta-analysis reviewed 16 studies that were conducted over the course of 25 years and together included more than 1.3 million people and over 100,000 deaths. The researchers discovered that people who frequently slept for less than six hours a night were more likely to die prematurely.

Perhaps more surprisingly, people who slept more than the recommended seven to eight hours per night were not immune to these negative effects. In fact, people who regularly slept more than eight or nine hours per night had a 30 percent higher risk of dying prematurely.

Bottom line? While the right amount of sleep may vary from person to person, in general humans fare best when they make like Goldilocks: Don’t sleep too little or too much, but instead find the sleep duration that is just right.

What Happens in the Brain and Body During Sleep?

Even though sleep scientists are still working to understand exactly why we sleep, there’s one thing they can all agree on: Sleep affects the brain and body in unique ways that aren’t replicated when we’re awake.

In order to understand these effects, it’s helpful to have a basic understanding of sleep stages. Here’s a general overview:

  • Human brains cycle through five distinct phases as they sleep: stage 1, 2, 3, 4, and 5.
  • Stages 1, 2, 3, and 4 are characterized by non-REM (rapid eye movement) sleep. These phases help restore both the body and brain in different ways. For example, some of these phases will facilitate learning and memory retention, while others help restore energy so you wake up feeling rested and ready to take on the day.
  • Stage 1 is characterized by light sleep that falls somewhere in between being awake and being in a deeper sleep. It typically lasts for a few minutes and involves several physiological changes, including slower heartbeats, breathing, eye movements, and brain waves.
  • Stage 2 involves a progression into a still-light-but-deeper state of sleep. Body temperature drops a bit, your eye movements stop, your muscles relax, and you’re no longer aware of your surroundings. This stage will repeat more than any other stage throughout a series of sleep cycles.
  • Stages 3 and 4 are characterized by deep, restorative sleep. These are the stages that enable you to feel alert and energized when you wake up. Brain waves slow down, heartbeats and breathing continue to slow, and muscles remain relaxed. These stages occur for the longest periods in the first half of the night. These stages are sometimes referred to as SWS, or Slow Wave Sleep. Two groups of cells in the brain stem switch on to trigger SWS.
  • Stage 5 is characterized by REM sleep, which accounts for approximately 25 percent of a person’s sleep throughout the night. Somewhere between 70 and 90 minutes after falling asleep, you’ll enter REM sleep. After the first REM sleep cycle, you’ll experience more rounds of REM sleep at intervals of approximately 90 to 110 minutes. This stage involves more active brain wave activity, faster breathing, and increased heart rate and blood pressure. This is also the stage during which you’re most likely to dream.
  • As the night goes on, periods of deep sleep become shorter in duration while periods of REM sleep become longer in duration. Near the end of a full night’s sleep, most of a person’s sleep will take place in stages 1, 2, and REM.

As the preceding information indicates, these different sleep stages are all correlated to different changes in the body and mind. The most significant stages in this regard are generally thought to be stages 3, 4, and 5 (or REM).

Even before falling asleep, the brain begins to undergo a few changes. Sleep-promoting neurons get more active prior to bedtime, while neurotransmitters start to limit the activity of cells that typically stimulate arousal and encourage the activity of cells that are associated with relaxation.

During stages 3 and 4, your brain and body undergo the following changes:

  • Blood pressure drops.
  • Blood supply to the muscles increases.
  • Brain waves slow down.
  • Breathing rate slows down.
  • Energy is restored.
  • Hormones are released.
  • Muscles relax.
  • Proteins in the brain are synthesized.
  • Tissues are repaired.

During stage 5 or REM sleep, your brain and body both get in on the action in different ways:

  • The brain and body both feel more energized.
  • The brain becomes more active. (In fact, electrical activity in the brain during REM sleep is not unlike electrical activity when a person is awake, except that the body is unmoving.) As noted above, this is the stage of sleep during which dreaming is most likely to occur.
  • The eyes move from side to side.
  • Aside from the eyes, the body is relaxed and unmoving.
  • Your body stops closely regulating body temperature. This can make your body temperature more susceptible to external factors, such as the temperature of your bedroom or type of mattress you’re sleeping on.

More generally, several other cognitive and physiological changes take place during sleep. (Some of these changes are better understood than others.) For example:

  • It’s possible the brain clears out waste products during sleep. Specifically, cerebral spinal fluid flushes through the brain to clean out waste chemicals. This helps freshen up the brain so it can continue to function at its best whenever you’re awake.
  • As we sleep, our brains process and store memories and information that we encountered and/or learned during the day. As a general rule, this info is transferred from short-term memory to long-term memory. This helps explain why research has found that sleep deprivation can impair learning and memory.
  • Brain glycogen levels are restored. These levels tend to fall during the day, which partly explains why our energy is lower by the time we hit the sheets. Restoring glycogen in the brain during sleep helps ensure that we wake up feeling rested and re-energized.
  • Immune system function is improved. As you sleep, your immune system releases cytokines—little compounds that help protect against inflammation and infection. If you don’t sleep enough, your body won’t produce enough cytokines to sustain healthy immune function.

How Does Sleep Support Overall Health?

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How Does Sleep Support Overall Health?

Once you understand a bit about what’s happening in the body and brain during sleep, it’s easy to see how sleep can support overall health. Sleep impacts nearly every system and tissue in the body, which helps explain why research suggests adequate, high-quality sleep provides the mind and body with a huge range of benefits. For example, good sleep has been shown to:

  • Improve memory and learning
  • Stimulate creativity
  • Increase capacities for focus and concentration  
  • Improve problem-solving and decision-making abilities
  • Improve workplace performance
  • Lower stress levels and improve mood overall
  • Reduce the risk of anxiety and/or depression
  • Improve academic performance
  • Replenish energy
  • Fight off inflammation
  • Improve athletic performance
  • Sustain a healthy weight and stave off unhealthy weight loss or gain
  • Reduce the risk of accidents and/or injury on the road, on the job, and elsewhere
  • Reduce both acute and chronic pain
  • Improve sex drive and the overall quality of a person’s sex life
  • Enhance immune system function
  • Reduce the risk of both less serious health issues (such as the common cold or flu) and more serious health issues (including diabetes, heart disease, and stroke)
  • Improve social skills and reduce social conflict stemming from irritability, stress, and so on
  • Enhance overall quality of life
  • Improve the likelihood of living a longer life

Even though research consistently affirms that high-quality sleep is paramount for sustaining virtually every aspect of our wellbeing, studies suggest many Americans still aren’t getting enough of it. We’ll touch on the consequences of sleep deprivation in the next section.

What Happens If We Don’t Get Enough Sleep?

As noted above, too many Americans are walking around sleep-deprived. Research suggests up to a third of working U.S. adults obtain less than six hours of sleep each night. That means they’re at risk for all the negative consequences that go along with sleep deprivation.

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What Happens If We Don’t Get Enough Sleep?

As the term implies, sleep deprivation occurs when someone is deprived of sleep—i.e., when they don’t get enough sleep or enough high-quality sleep. Sleep deprivation can be caused by a number of factors ranging from illness to sleep disorders, family or work obligations, certain medications or medical issues, a poor sleeping environment (such as a bright or noisy bedroom), and poor sleep habits or lifestyle choices (such as drinking caffeine before bed or going to bed later than would allow for obtaining adequate sleep).

No matter the cause, sleep deprivation can result in a cascade of negative consequences. Here are some of the unpleasant side effects that can occur when a person doesn’t get enough sleep:

  • It impairs the healthy function of the body’s main systems, including the cardiovascular, central nervous, digestive, endocrine, immune, musculoskeletal, and respiratory systems.
  • It increases the risk of infection and illness as well as the risk of serious health conditions such as cancer, diabetes, heart disease, stroke, and overall mortality.
  • It increases the risk of anxiety, depression, and other mental illnesses.
  • It increases the risk of injury or fatality stemming from car, workplace, or at-home accidents.
  • It increases the likelihood of unintended weight loss or gain.
  • It increases the risk of developing sleep disorders such as insomnia, narcolepsy, or sleep apnea.
  • It impairs workplace performance.
  • It interferes with personal relationships and may inhibit a healthy social life.

Not surprisingly, failing to get enough sleep on a regular basis is associated with reduced quality of life overall.

The Bottom Line On Why We Sleep

Sleep researchers are still working to understand exactly why we sleep. Over the years, several theories have emerged to explain the phenomenon of sleep—these include the Inactivity Theory, the Energy Conservation Theory, the Restorative Theory, the Brain Plasticity Theory, and the Reproductive Fitness Theory. Most likely, a combination of each of these theories helps explain why humans need to sleep every day.

No matter the reason behind why we sleep, sleep experts all agree that regularly obtaining adequate amounts of high-quality sleep is essential for total health and wellness—from physical performance to cognitive function and emotional wellbeing. Not getting enough sleep, on the other hand, is associated with a host of negative consequences that can impair a person’s physical, cognitive, emotional, and social functioning.

Bottom line? While researchers continue to explore exactly why it is that humans evolved to sleep on a daily basis, it’s a good idea to go ahead and make sure you’re getting seven to eight hours of shut-eye every night.

Logan Block

Logan is the former content director of Sleepopolis. As content director, he reviewed new mattresses every week and curated the comparisons, best of pages, and video guides on the site. Logan perfected his method by personally testing over 200 different mattresses, so he’s not only able to discern the overall vibe of a specific bed, but to contextualize its feel within the bed-in-a-box market as a whole.