While it's something you have to do every day, why we sleep remains a mystery. Many theories about sleep abound, but scientists are far from universal agreement on how to answer the question, "Why do we sleep at night?" Only in the last few decades have they begun to unlock the true secrets of sleep. At least four common theories are at play, but it remains to be seen which, if any, are correct.
You might look at these theories and the scientific evidence behind them and wonder why not all of them, at least in part, might be correct. They could be, but researchers continue to look for "the" main reason for sleeping, with many subscribing to a belief expressed in a 1998 paper:
all the righteous
It can be understood that sleep serves many different functions, but intuition suggests that there is one essential function. The discovery of this function will open an important door to the understanding of biological processes.
— All the righteous
The sleep cycle involves several stages, but these theories usually focus onrapid eye movement (REM) sleep- which is when youto dream— and the other stages grouped together as non-REM sleep.
The 4 stages of sleep
Restorative Theory
The restorative theory of sleep, first proposed in 2006, is among the most accepted explanations for why people need sleep. This suggests that the purpose of sleep is to store memory and restore our brains and bodies for the next day.
- Organize and store memories.: Memories are believed to be converted from short-term storage to long-term storage, while information deemed unimportant is removed. This mostly occurs during REM sleep, which is when brain cells most involved in memory, attention, and learning are least active.
- Cleanses and replenishes brain chemicals: Many brain chemicals build up while you're awake, includingadenosine, which makes you drowsy when it builds up. Meanwhile, during sleep, the brain "replenishes" the chemicals it uses for signaling and other purposes, so it has enough for the next day.
- Elimination of toxic waste from the brain.: Like chemicals in the brain, waste products from energy metabolism accumulate during the day and are eliminated at night. (However, the main evidence for this comes from studies in mice, not humans.)
- Tissue repair throughout the body.: During sleep, your body increases cell division,growing hormonelevels and production of proteins necessary for proper functioning. This is believed to be the reason why strenuous physical activity during the day increases REM sleep time.
- rest for the brain: Inactivity duringnon REM sleepseems to provide a rest period for the brain.
Some of the brain chemicals that build up during the day are associated with the plaques that characterizeAlzheimer's disease, so it's theorized that sleep brain-clearing activities may protect you against Alzheimer's disease.
| Sleep recommendations for adults | |
|---|---|
| Age | hours |
| 18-60 | 7+ |
| 61-64 | 7-9 |
| 65+ | 7-8 |
adaptive theory
Also called evolutionary theory or preservation theory, the original version of this early theory suggested that, as humans evolved, hiding at night increased our ability to survive.
For our early ancestors, the night was dangerous, especially since predators that hunt at night do better in the dark than humans do, so it made sense to find a safe haven. Also, since they couldn't go out and find food on their own, their bodies slowed down to save energy for when they could be active. People who avoided danger in this way, the theory argued, lived longer and were more likely to reproduce.
So sleep became an adaptive or evolutionary advantage and became part of the species' neurochemistry. However, most experts reject this idea because sleep leaves animals (including humans) vulnerable and helpless, which doesn't support the idea that sleep made our ancestors safer.
One flaw in this theory, some experts say, is that sleep limits productivity, such as finding food and reproducing, so staying awake longer would be an evolutionary benefit. However, no species has evolved without the need for sleep, casting doubt on the adaptive benefit.
According to some sleep researchers, the daily need for sleep combined with the encouragement not to be outdoors in the dark caused us to adapt to function better during the day, which prevented us from adapting to the dark.
how to sleep better
Theory of Conservation of Energy
Similar to aspects of other theories, some experts theorize that the main purpose of sleep is to save energy. By sleeping, they say, you can spend some of your time running with a lower metabolism.
This reduces the number of calories you need to eat. For early humans, this extra need for food could have been the difference between life and death, or the survival of the species in the face of extinction. It was harder to gather food at night, so it made sense to stay hidden at that time. They also point to the brain's need to replenish its stores of glycogen, which is an important fuel.
However, while it is true that metabolism slows down during non-REM sleep, the brain is extremely active during REM sleep, which some say is a blow to the theory of energy conservation.
10 benefits of a good night's sleep
brain plasticity theory
Among the most recent theories isbrain plasticity(also called neuroplasticity), which is the brain's ability to change and adapt in response to experience. You can change both the functional aspects (like relearning skills in a new area after damage) and the structural aspects (like forming new paths due to learning).
Brain plasticity theory says that sleep is necessary for the brain to make structural changes. Support for this theory comes from many quarters.
Similar to restorative theory, this concept deals with information processing and memory formation. Research suggests that sleep loss leads to decreased structural plasticity, which can have a negative effect on alertness, cognition and mood.sleep deprivationit also compromises memory formation, which is related to learning and plasticity.
Plasticity theory is thought to explain why babies and young children need a lot of sleep: they are learning so much about the world that their brains need more time to process it. Researchers are even trying to promote less sleep interruptions for premature babies in neonatal intensive care, citing studies of the long-term impact of sleep on brain development and plasticity.
Some researchers have even theorized that sleep is the price we pay for brain plasticity. This concept is based on the importance of the processes that occur during sleep for the brain's ability to adapt and change.
Decreased sleep needs
Newborns need 14 to 17 hours of sleep a day. The recommended amount of sleep decreases throughout childhood, with teenagers needing 8 to 10 hours a day.
Common questions
Why do we close our eyes when we sleep?
There are several reasons why most people sleep with their eyes closed, although some people actually sleep with their eyes open. Closed eyes remain moist and are protected by the eyelids during sleep. The eyelids also block light during sleep, and light signals the brain that it's time to wake up.
At what stage of sleep can you have sleep paralysis?
Paralysis I am, or the feeling that you are aware but unable to control your body, is very common and usually occurs during the transition from REM to the waking state.
Why do we dream?
There are many theories that explain why we dream, such as that dreaming helps with memory processing and allows us to process emotions.Sigmund Freud believed that dreams express our unconscious desires and deepest desires.
A Word from Verywell
While it's a phenomenon we don't fully understand, sleep is critical to our everyday health. Not only necessary for restoration and repair, learning and memory, growth and development and brain plasticity, sleep also helps with problem solving, healthy metabolism, blood sugar and hormone regulation, heart health and strengthening immunity.With how crucial it is to our survival, it's no wonder so many of us want more.
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Raven F, Van der Zee EA, Meerlo P, Havekes R.The role of sleep in regulating structural plasticity and synaptic strength: implications for memory and cognitive function..sleep like rev. 2018;39:3-11. doi:10.1016/j.smrv.2017.05.002
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ByDr. Brandon Peters
Brandon Peters, MD, is a board-certified neurologist and specialist in sleep medicine.
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FAQs
What are the two 2 major theories about why we sleep? ›
One evolutionary theory of sleep holds that sleep is essential for restoration of resources that are expended during the demands of day-to-day life. A second theory proposes that our sleep patterns evolved as an adaptive response to predatory risks, which increase in darkness.
What are two theories related to sleep and dreaming? ›Physiological theories claim that dreams are a product of some kind of existing disorder in the brain. Psychological theories claim dreams are a way of processing issues in our lives. Physiological theories claim that dreams are a product of processes in the body that the brain interprets when we sleep.
What is the theories of sleep deprivation? ›Lapse and wake-state instability hypotheses
This theory suggested that baseline levels of functioning are identical in rested and sleep-deprived states, but that sleep-deprived individuals experience transient phases of low arousal during which sleep intrusions and performance lapses occur.
Oswald said that sleep is important for restorative purposes. According to him, sleep deprivation can result in mental problems and even death. Oswald's theory was that we constantly expend energy and need to be replenishing this energy to restore ourselves.
What was the first theory of sleep? ›In fact, one of the first scientific theories about sleep dates from around 500 to 450 B.C. Alcmaeon of Croton, a Greek doctor and philosopher, postulated that sleep is the result of blood on the surface of the body withdrawing into the interior.
What are the 3 main theories of dreaming? ›There are three major theories: the psychodynamic theory of dreaming, the physiological theory of dreaming, and the cognitive theory.
Which theory is the best explanation for the function of dreams? ›Dreams Process Information
According to the activation-synthesis model of dreaming, which was first proposed by J. Allan Hobson and Robert McCarley, circuits in the brain become activated during REM sleep, which triggers the amygdala and hippocampus to create an array of electrical impulses.
A restorative theory claims that sleep is used to repair the body including the brain. Oswald suggests that slow wave sleep is when body repair occurs and REM sleep is when the brain is repaired. This is supported by the fact that there is an increase in the secretion of growth hormones during SWS.
What are the 4 theories of sleep? ›Several prominent theories have explored the brain and attempt to identify a purpose for why we sleep, which includes the Inactivity theory, Energy conservation theory, Restoration theory, and the Brain plasticity theory.
What are the 5 functions of sleep? ›Nevertheless, it is quite evident that sleep is essential for many vital functions including development, energy conservation, brain waste clearance, modulation of immune responses, cognition, performance, vigilance, disease, and psychological state.
What did Aristotle say about sleep? ›
According to Aristotle, sleep occurs as a consequence of the digestive process regulated by the nutritive part of the soul.
What is Sigmund Freud's theory? ›In simple terms, Sigmund Freud's theory suggests that human behavior is influenced by unconscious memories, thoughts, and urges. This theory also proposes that the psyche comprises three aspects: the id, ego, and superego. The id is entirely unconscious, while the ego operates in the conscious mind.
What is the purpose of cognitive theory? ›Cognitive theory seeks to understand human learning, socialization, and behavior by looking at the brain's internal cognitive processes. Cognitive theorists want to understand the way that people process information.
What is the major theory of dreams? ›Freud believed that dreams, by nature, disguised their meaning. In contrast, Jung believed that dreams were actually direct expressions of the mind itself. Dreams, he thought, expressed an individual's unconscious state through a language of symbols and metaphors.
What are the two discussed theories used to explain dreams? ›The self-organization theory of dreaming offers a framework distinct from psychoanalytic theories to explain how dreams are generated and operate.
What is the purpose of dreams sleep? ›The evidence points toward an important function of dreams: to help us take the sting out of our painful emotional experiences during the hours we are asleep, so that we can learn from them and carry on with our lives.
What is Kaplan's attention restoration theory? ›Attention Restoration Theory (ART) (Kaplan, 1989, 1995) suggests that mental fatigue and concentration can be improved by time spent in, or looking at nature. The capacity of the brain to focus on a specific stimulus or task is limited and results in 'directed attention fatigue'.
What are the restoration and adaptive theories of sleep? ›Repair and Restoration Theory of Sleep
According to it, sleep affords the body the much needed opportunity it requires to repair and rejuvenate itself, which involves restoring and revitalizing the physiological processes which keep your body and mind healthy and functioning properly.
Repair theory is a attempt to explain how people learn procedural skills with particular attention to how and why they make mistakes (i.e., bugs). The theory suggests that when a procedure cannot be performed, an impasse occurs and the individual applies various strategies to overcome the impasse.
What are the two main theories of dreaming? ›Freud believed that dreams, by nature, disguised their meaning. In contrast, Jung believed that dreams were actually direct expressions of the mind itself. Dreams, he thought, expressed an individual's unconscious state through a language of symbols and metaphors.
What are the 2 components of sleep regulation? ›
Researchers explain that sleep regulation has two components: Build-up of homeostatic sleep drive. Circadian rhythms.
What are the 2 main types of sleep? ›There are two basic types of sleep: rapid eye movement (REM) sleep and non-REM sleep (which has three different stages).
What are 3 theories of why we sleep? ›Several prominent theories have explored the brain and attempt to identify a purpose for why we sleep, which includes the Inactivity theory, Energy conservation theory, Restoration theory, and the Brain plasticity theory.
What are the 3 major dream theories? ›There are three major theories: the psychodynamic theory of dreaming, the physiological theory of dreaming, and the cognitive theory. The physiological theories of dreaming discuss how the brain processes information, and how it manifests as a dream.
What are the four theories that explain dreams? ›This section discusses the main assumptions, hypotheses, and findings with regard to four other contemporary theories of dreaming—Freudian theory, activation-synthesis theory, memory-consolidation theory, and threat-simulation theory.
What are the 5 purposes of sleep? ›Nevertheless, it is quite evident that sleep is essential for many vital functions including development, energy conservation, brain waste clearance, modulation of immune responses, cognition, performance, vigilance, disease, and psychological state.
What are the 5 main reasons for sleep? ›- It's your body's reset process. ...
- It balances your hunger cues. ...
- It restores your creativity, productivity and concentration. ...
- It alleviates stress and anxiety. ...
- It strengthens the immune system. ...
- Want to keep dreaming?
Sleep recharges the brain, allowing it to learn and make memories. Insufficient sleep has been linked to car crashes, poor work performance and problems with mood and relationships. Sleep deprivation also raises the risk of high blood pressure, heart disease, diabetes, obesity, depression, and stroke.
What are some scientific facts about sleep? ›If we sleep too little, we become unable to process what we've learned during the day and we have more trouble remembering it in the future. Researchers also believe that sleep may promote the removal of waste products from brain cells—something that seems to occur less efficiently when the brain is awake.
What are the stages of sleep in psychology? ›Sleep occurs in five stages: wake, N1, N2, N3, and REM.
What happens to your body when you fall asleep? ›
Your body temperature decreases, your eye movements stop, and your heart rate and muscles continue to relax. Your brain waves briefly spike then slow down.
What part of the brain is responsible for sleep? ›This circuit originates in a brain structure known as the thalamic reticular nucleus (TRN), which relays signals to the thalamus and then the brain's cortex, inducing pockets of the slow, oscillating brain waves characteristic of deep sleep.
Who is the man who doesn't sleep? ›Al Herpin (January 1, 1862 in Paris – January 3, 1947) was an American known as the "Man Who Never Slept". Al Herpin, who lived in Trenton, New Jersey, claimed to have never slept. The supposed cause is unknown.
What are the 4 stages of sleep? ›There are traditionally 4 stages of sleep: awake, light, deep, and REM sleep. Each one plays an essential role in maintaining your mental and physical health. As you're reading about sleep, you may also see the terms “NREM” or “NREM Stages 1-4.” These are simply other terms for the phases of sleep.