Restorative Theory and More Ideas About Why We Sleep

how sleep restores the brain

Verywell / Hetal Rathod

Even though it's something you have to do every day, why we sleep remains a mystery. Many sleep theories abound, yet scientists are far from universal agreement about how to answer the question, "Why do we sleep at night?" Only in the last few decades have they even begun to unravel sleep's true secrets. At least four common theories are in play, but it remains to be seen which—if any—are correct.

You may look at these theories and the scientific evidence that supports them and wonder why they all can't, at least in part, be right. They could be, but researchers are continuing to search for "the" core reason for sleep, and many subscribe to a belief expressed in a 1998 paper:

Allen Rechtschaffen

Sleep can be understood as fulfilling many different functions but intuition suggests there is one essential function. The discovery of this function will open an important door to the understanding of biological processes.

— Allen Rechtschaffen

The sleep cycle involves several stages, but these theories generally focus on rapid-eye movement (REM) sleep—which is when you dream—and the other stages lumped together as non-REM sleep.

Restorative Theory

The restorative theory of sleep, first proposed in 2006, is among the more accepted explanations for why people need sleep. It suggests that the purpose of sleep is to store memory and restore our brains and bodies for the next day.

  • Organizing and storing memories: Memories are believed to be converted from short-term to long-term storage, while information deemed unimportant is removed. This primarily occurs during REM sleep, which is when the brain cells most involved in memory, attention, and learning are least active.
  • Clearing out and replenishing brain chemicals: Many brain chemicals build up while you're awake, including adenosine, which makes you sleepy when it accumulates. Meanwhile, during sleep, the brain "restocks" the chemicals it uses for sending signals and other purposes, so you have enough for the next day.
  • Clearing waste toxins from the brain: Similar to brain chemicals, the waste products of energy metabolism build up during the day and are cleared out at night. (However, the primary evidence for this comes from studies of mice, not humans.)
  • Repairing tissues throughout the body: During sleep, your body increases cellular division, growth hormone levels, and the production of proteins necessary for proper function. This is believed to be why strenuous physical activity during the day increases your time in REM sleep.
  • Rest for the brain: The inactivity during non-REM sleep appears to provide a period of rest for the brain.

Some of the brain chemicals that accumulate during the day are associated with the plaques that characterize Alzheimer's disease, so it's theorized that the brain-clearing activities of sleep may protect you against Alzheimer's.

Adult Sleep Recommendations
Age Hours
 18-60 7+
 61-64 7-9 
 65+ 7-8 
Source: U.S. Centers for Disease Control and Prevention

Adaptive Theory

Also called the evolution theory or preservation theory, the original version of this early theory suggested that while humans evolved, hiding out overnight increased the ability to survive.

For our early ancestors, nighttime was dangerous—especially because the predators who hunt at night function better in the dark than humans do—so it made sense to seek a safe refuge. Also, because they couldn't be out finding food themselves, their bodies slowed down to conserve energy for when they could be active. People who avoided dangers in this way, the theory argued, lived longer and were more likely to reproduce.

Thus, sleep became an adaptive or evolutionary advantage and became part of the neurochemistry of the species. However, most experts reject this idea because sleeping leaves animals (including humans) vulnerable and defenseless, which doesn't support the idea that sleeping made our ancestors safer.

A flaw in this theory, some experts say, is that sleeping limits productivity, such as finding food and reproducing, so staying awake longer would be an evolutionary benefit. However, no species evolved without a need for sleep, which calls the adaptive benefit into question.

According to some sleep researchers, the daily need for sleep combined with the incentive not to be out in the dark caused us to adapt to function best during daylight, which prevented us from adapting to the dark.

Energy Conservation Theory

In a similar vein to aspects of other theories, some experts theorize the primary purpose of sleep is conserving energy. By sleeping, they say, you're able to spend part of your time functioning at a lower metabolism.

That lowers the number of calories you need to eat. For early humans, that extra food requirement could have been the difference between life and death, or survival of the species versus extinction. It was harder to gather food at night, so it made sense to stay hidden then. They also point to the brain's need to replenish its reserve of glycogen, which is an important fuel.

However, while it's true that metabolism slows during non-REM, the brain is extremely active during REM sleep, which some say is a strike against the energy conservation theory.

Brain Plasticity Theory

Among the more recent theories deals with brain plasticity (also called neuroplasticity), which is the brain's ability to change and adapt in response to experience. It can change both functional aspects (such as re-learning skills in a new area after damage) and structural aspects (such as forming new pathways due to learning).

The brain plasticity theory says that sleep is necessary for the brain to make structural changes. Support for this theory comes from many places.

As in restorative theory, this concept deals with information processing and memory formation. Research suggests that sleep loss leads to less structural plasticity, which can have a negative effect on alertness, cognition, and mood. Sleep deprivation also compromises memory formation, which is related to learning and plasticity.

It's believed that the plasticity theory explains why babies and young children require a lot of sleep—they're learning so much about the world that their brains need more time to process it. Researchers are even trying to promote less sleep interruption for preterm babies in neonatal intensive care, citing studies about sleep's long-term impact on brain development and plasticity.

Some researchers have even theorized that sleep is the price we pay for brain plasticity. That concept is based on the importance of the processes that occur during sleep to the brain's ability to adapt and change.

Declining Sleep Needs

Newborns need between 14 and 17 hours of sleep per day. The recommended amount of sleep declines throughout childhood, with teenagers needing between 8 and 10 hours a day.

Frequently Asked Questions

Why do we close our eyes when we sleep?

There are several reasons why most people sleep with eyes closed, though some people do actually sleep with their eyes open. Closed eyes stay moist and are protected by the eyelids during sleep. Eyelids also block out light during sleep, and light signals to the brain that it is time to wake up.

At what stage of sleep can you experience sleep paralysis?

Sleep paralysis, or the feeling that you are conscious and yet unable to control your body, is very common and typically occurs during the transition from REM to being awake.

Why do we dream?

There are a lot of theories explaining why we dream, such as that dreaming aids in memory processing and that it allows us to process emotions. Sigmund Freud believed that dreams express our unconscious desires and deepest wishes.

A Word From Verywell

While it's a phenomenon we don't fully understand, sleep is critical to our daily health. Not only is it necessary for restoration and repair, learning and memory, growth and development, and brain plasticity, sleep also helps with problem-solving, a healthy metabolism, blood-sugar and hormone regulation, heart health, and strengthening immunity. With how crucial it is to our survival, it's no wonder many of us long for more of it.

11 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Brandon Peters, MD
Brandon Peters, MD, is a board-certified neurologist and sleep medicine specialist.