Why Do We Need to Sleep? Here’s What the Science Says

Perhaps you’ve never really stopped to think about something we do every night of our lives: why do we need sleep?

Simply put, and unsurprisingly, it’s essential for good health—but in more ways than you may think. Sleep is essential to recharge your body and mind so you can function at your best. And generally, most adults need between seven and nine hours of sleep.

“Sleep is a vital part of our daily lives, yet 70 million Americans suffer from a sleep disorder—and approximately 95 percent of those affected remain undiagnosed and untreated,” says Lauri Leadley, clinical sleep educator and president and owner at Arizona’s Valley Sleep Center.

Sleep has a real impact on your overall health, shares Leadley.

“When we are sleep-deprived, it impacts everything from our physical body to our ability to function to our mental health,” she says. “It only takes one night of lost sleep to impair our judgment equal to that of a drunk driver, according to the National Sleep Foundation.”

Since Leadley says getting enough sleep should be as high on your priority list as exercising and eating right, ahead, we’ll delve into the specific reasons why sleep is so important and answer commonly asked questions about sleep.

Why do we need sleep?

While there are still a lot of unknowns when it comes to the purpose of sleep, experts do have some theories that help answer the question, “Why do we need sleep scientifically?”

At its core, Leadley explains that sleep is “the only time our body has time to heal,” calling it nature’s method of recharging the body.

Conserve your energy

Sleep is the body’s way of hitting the reset button. As Leadley says, sleep “refreshes your energy to prepare you to meet the challenges of the next day.” The energy savings gained during sleep have been studied and proven through scientific research.

Maintain your brain function

Sleep is an effective way to sharpen thinking, improve memory and retention, and enhance productivity, according to Leadley. In a way, sleep is a way to wipe the brain’s slate clean so one can function better the following day.

“It is possible that one of the essential functions of sleep is to take out the garbage, as it were, erasing and ‘forgetting’ information built up throughout the day that would clutter the synaptic network that defines us,” says one 2017 study. “It may also be that this cleanup function of sleep is a general principle of neuroscience, applicable to every creature with a nervous system.”

Regulate your emotions

Sleep is crucial for regulating emotions, reducing stress, and maintaining mood and good mental health, with studies often linking difficulty sleeping to poor mental well-being.

“The amount of sleep you get and your overall mood are very closely related,” Leadley states. “Everyone knows that one of the surest signs that someone is overtired is that they are irritable and crabby.”

Getting the sleep you need can help stabilize your mood while also decreasing the likelihood of certain mental health conditions, Leadley adds.

“Research indicates that sleep deprivation increases your risk of some mental health conditions like depression,” she says.

Boost your immunity

Scientists are finding more and more that there’s a direct connection between sleep quality and immunity. One 2017 study shows changes in the sleep-wake cycle and immune modulators are linked, which can impact sleep regulation and immune response.

During sleep, there’s a lot going on in your body, says Leadley.

“One of the most important things your body does while you are sleeping is boosting and improving your immune system,” she explains. “If you don’t get enough sleep, it degrades your immune system’s ability to function optimally, making you more susceptible to illnesses of all types from the flu to cancer.”

Protect your heart

Leadley says good sleep can result in better heart health, lowered blood pressure, and a lower risk of heart attacks and heart disease, something that’s been additionally backed up by scientific research.

“We all know how important it is to promote and protect our cardiovascular health, but not many of us realize the direct connection between cardiovascular health and sleep,” Leadley says. “Not getting enough sleep can contribute to high blood pressure and other cardiovascular system conditions.”

Researchers also believe lack of sleep may increase the risk of dying from a heart attack, she adds.

Prevent diabetes

Research cited by the National Sleep Foundation indicates there’s an association between hormonal changes resulting from sleep loss and an increased risk of developing type 2 diabetes, notes Leadley.

What happens when you sleep?

So, why do we need REM sleep? And why does our body continually cycle through stages of sleep throughout the night? Here, Leadley explains what happens during each stage of sleep:

The beginnings of sleep (pre-sleep)

As your body relaxes and you settle in to fall asleep, your brain activity displays a small, fast wave pattern called beta waves. After several minutes, you enter that twilight-type state of almost asleep as your body continues to relax and your brain switches to alpha waves.

During this timeframe, you may experience hypnogogic hallucinations like feeling as if you’re falling or hearing someone call your name. Myoclonic jerks, the random, sudden, startled movements of a body part, also happen during this pre-sleep stage.

Stage 1: light sleep

Light sleep, the transition from being awake to being asleep, is the first official stage of sleep and the beginning of the sleep cycle. During this stage, your brain shows theta wave patterns on the electroencephalogram (EEG), a recording of the electrical activity in your brain.

Lasting about five to 10 minutes, someone awakened while in this light sleep stage may not even think they’ve slept. Stage 1 is only encountered at the beginning of the first sleep cycle when you fall asleep—and, unlike other stages, isn’t repeated throughout the night.

Stage 2: unconscious sleep

The second stage of sleep lasts for about 20 minutes and is the least active of the sleep cycle stages. Although your brain waves get much more rapid and are punctuated with rhythmic spindles, you appear completely unconscious during this stage.

Your body further relaxes, and your body temperature decreases as your heart rate slows down. This is the first stage where the brain and body begin to have a divergent experience, with the brain becoming more active as the body becomes more passive.

Stage 3: deep sleep

The third stage of sleep is the transition from light sleep to the deepest type of sleep. Your brain activity shows the deep, slow pattern of delta waves during this 20- to 30-minute stage.

Stage 4: deeper sleep

Often called delta sleep because of the delta wave brain activity pattern captured on the EEG, the fourth stage lasts for about 30 minutes, and sleepers in this stage can be very difficult to wake up.

Parasomnias like bedwetting, night terrors, and sleepwalking generally occur during this part of the sleep cycle. This is the last of the N-REM (non-rapid eye movement) sleep stages.

Stage 5: REM sleep

This stage is called REM sleep because of the rapid eye movements that can be observed while you’re in this part of the cycle. When you enter REM sleep, your respiration and brain activity increases. Your brain wave patterns resemble those captured when you’re awake.

This is the stage where the majority of dreaming occurs. Also called paradoxical sleep, during this stage, your brain and body have paradoxical experiences. As your brain activity increases, your body relaxes and all voluntary muscles become paralyzed.

Because REM sleep and wakefulness are so similar from the perspective of your brain, this paralysis, called atonia, is necessary to ensure your body doesn’t act out what your brain is dreaming.

The first REM stage is very short, lasting only a few minutes—but throughout the night, each REM stage will last longer than the previous one, shortening the other stages. By the end of the night, your last REM stage may last as much as 60 minutes.

Your body will continue to cycle throughout the night—but as Leadley points out, contrary to popular belief, people don’t experience sleep stages in a continuous, linear fashion.

“The first cycle generally proceeds from stage 1 to 4 in order, but most people will go back up to stage 3 and 2 before jumping to stage 5 for the first time,” she says.

It usually takes about 90 minutes to reach the first REM stage. “Once the first cycle is complete, you will generally bounce directly back to stage 2 to start the next cycle,” says Leadley. An average night’s sleep repeats a cycle pattern four to five times, she adds.

How much sleep do you need?

You may be wondering, “Why do we need eight hours of sleep?” Well, the answer is a bit more complicated than you may think.

“There are individual differences in sleep needs, but sleep also varies by age,” says Don Townsend, PhD, diplomate, behavioral sleep medicine and fellow, American Academy of Sleep Medicine.

Here are the recommended guidelines for sleep, according to age:

  • Babies: Experts recommend 14 to 17 hours of sleep per day for a baby.
  • Children: Additionally, experts advise school-age children get nine to 11 hours of sleep every day.
  • Teens: According to Townsend, teens often need eight to 10 hours of sleep “but don’t get nearly that much.”
  • Adults: Adults need an average of eight hours of sleep (or seven to nine hours). “There are some controversies about whether older adults under 65 need the same or less sleep or just that their sleep gets redistributed over the typical 24-hour day due to less activity and the other factors that affect older adults, including health issues,” Townsend observes.
  • Seniors: The National Institute on Aging says those over age 65 should aim for seven to nine hours of sleep.

What happens if you don’t get enough sleep?

There are clear side effects that can be felt when you don’t get enough sleep. These signs can include:

  • Anxiety
  • Poor memory
  • Risk of certain diseases
  • Moodiness
  • Irritability
  • Depression
  • Stress
  • Low productivity
  • Low motivation
  • Forgetfulness
  • Inability to concentrate
  • Weight gain
  • Increased appetite
  • Lowered sex drive
  • Lethargy during the day, which can include yawning or needing coffee after coffee

“If you’ve ever gotten out of bed thinking you can’t wait for the day to be over so you can get back in it, chances are you’re not getting enough sleep,” says Leadley.

The bottom line: “Sleep is integral to wellness,” says Leadley. “It impacts every aspect of life—mind, body, and spirit.”

If you can’t sleep—or you struggle to fall asleep, stay asleep, and don’t look forward to those restful hours—consider signing up for a sleep study to see if there’s a deeper issue, suggests Leadley.

“A diagnosis and treatment plan can vastly improve your sleep health—and whole life,” she says.


What are three reasons why sleep is important?

Sleep is crucial for your overall wellness. Some of the top ways your life can be impacted by lack of sleep include lowered energy, poor mental health, and risk for developing certain diseases.

What happens if you don’t sleep?

Your mind and body will be quickly, and directly, affected. You’ll experience heightened stress, brain fog, and increased appetite, among several side effects.

Beyond physical sleep, there are other types of rest that can help you recharge. Here’s a rundown of the different types of rest and why they’re important.

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