Why Do We Sleep?

We all know that it’s important to get enough sleep if we want to perform at our best. Yet, scientists are still unable to answer the overarching question, why do we sleep?

By Nicole Gleichmann

May 5th, 2022

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We spend roughly one-third of our lives slumbering away, which begs the question, why exactly do we sleep? With sleep comprising such a large percentage of our lives, it seems like we should know the answer to this question…but as of yet, scientists are still hard at work trying to uncover the exact role of sleep.

We know that not getting enough hours of sleep is detrimental for our health. Sleep deprivation leads to memory troubles, mood difficulties, and a depressed immune system. And, per animal studies, if we didn’t sleep for an extended period of time, we would die.

While we understand that sleep is central to our health and wellbeing, we don’t have one straightforward answer as to why we sleep, or exactly what sleep does for us. In this article, we will delve into our understanding of sleep and the current scientific theories that try to answer the question: Why do we sleep?

What Happens When We Sleep?

For millennia it was believed that sleep was merely a passive state in which the body and mind rested or, as Aristotle believed, just an unremarkable and unimportant period marked by an absence of our usual sense perceptions. Sleep was thought of as a necessary evil or, at best, a kind of envelope or container for the more interesting and important act of dreaming.

However, it has become abundantly clear in recent decades is that sleep is not simply a matter of putting our mental and physical activities on hold for a while, but rather a genuine “second state” with its own complex and varied mental and physical activities. Although we do not have a definitive answer to the question “why do we sleep?,” with the recent increases in knowledge about exactly how sleep works, we are at least now in a better position to hazard some more educated guesses about just why we sleep.

Every night when you lie down to sleep, your body and brain undergo a variety of changes that are cued by your circadian rhythm, or your internal biological clock. In response to external and internal factors, your circadian rhythm dictates the release of hormones that encourage your brain to transition from wakefulness to a sleep state.

When you begin to fall asleep, your body temperature begins to drop as your brain waves slow. Your brain then cycles through the stages of sleep which consists of two types of sleep: non-rapid eye movements sleep, or non-REM sleep, and rapid eye movement sleep, or REM sleep.

During non-REM sleep, which consists of stage 1, 2, and 3 sleep, our brain activity slows and our brains experience slower brain waves. This sleep stage is central for our ability to recover from exercise and injury. REM sleep is different, with brain waves similar to those that occur when we’re sleeping. REM sleep is thought to be important for our mental health, including our long-term memory and learning abilities.

Each of these non-REM/REM cycles lasts about 90 minutes, and we continue to cycle through each stage while we sleep. In order to reap the full benefits of a night’s rest, it’s important for adults to experience a sleep duration around 7-8 hours each night.

Even though we understand much of what happens while we sleep, there is a lot left to learn. Plus, knowing some of what sleep does doesn’t exactly answer the question of why we sleep.

Current Scientific Theories on Why Humans Sleep

Over the years, many theories about the function of sleep have been put forward, several of which are briefly described in this section. The actual reason we sleep is likely to be a combination of most or all of them.

Theory #1: Energy Conservation

It used to be thought that the major function of sleep was to reduce wear and tear on the body and to conserve the body’s energy resources by allowing for a substantial period of downtime. During this time, energy could be saved and built up again, like recharging a battery. When we sleep, we burn far fewer calories than we do when we are awake, leading to the hypothesis that sleep might be a way to help us conserve energy and require less food.

This idea that sleep plays a role in conservation is supported by the fact that our brain’s glycogen levels (glycogen is the fuel that our brains use for energy) drop dramatically during the day and are replenished at night, suggesting that energy is replenished while we sleep. Additionally, it’s colder at night, so we would require more energy to stay awake during the night than during the day. By sleeping through the night, we don’t burn so many calories.

Body Temp Variation

Animals, particularly warm-blooded animals like mammals and birds, use a lot of energy to maintain body temperature and other bodily functions. The waking brain in particular uses a great deal of energy, which needs to be recuperated. During sleep, our core temperature is reduced by around 1°C, and so we use less energy maintaining our body temperature, suggesting that one of the primary functions of sleep may be to conserve energy.

The energy conservation theory is also supported by the fact that, in general, metabolic rates are higher in smaller animals, which typically sleep longer hours, and that cold-blooded animals like reptiles and fish tend to exhibit less obvious and unequivocal sleep.

Related: Sleep in the Animal Kingdom

Theory #2: Recovery and Restoration

A common theory that is gaining support is that we sleep to allow our bodies to heal and rejuvenate. It is now widely thought that sleep has a kind of “house-keeping” purpose, both mental and physical. Sleep seems to facilitate the repairing and renewing of tissues and nerve cells, the neutralization of neurotoxins, and the restoration of normal levels of chemicals throughout our bodies. Even more specifically, it appears that while REM sleep is largely devoted to brain repair and restoration, non-REM sleep is principally a time for body repair and restoration.

Sleep affects things like protein synthesis, muscle repair, and clearing out toxins that accumulate throughout the day. For example, a chemical called adenosine is produced in the brain during the day, and accumulation of adenosine can lead to our feeling fatigued. When we sleep, adenosine and other toxins are flushed from the brain and body. This allows for us to recover and start over the next morning.

The physical healing of wounds is expedited by sleep, and sleep strengthens the immune system in general. Rats deprived of sleep in experiments show distinctly inferior healing capacities, develop skin lesions, lose body mass, and are unable to maintain a stable body temperature, ultimately dying of sepsis or just “exhaustion.” Sleep-deprived rats have been shown to exhibit substantially fewer leukocytes (white blood cells), the body’s main defense against infection, and sleep-deprived humans show less than half of the protective antibodies after an inoculation jab as compared to people with healthy sleep patterns.

Sleep and growth hormone development

Sleep, especially stage 3 slow-wave sleep, has also been associated with increased levels of growth hormone levels in the body. Growth hormone is an important factor in tissue regeneration and repair. However, it is not necessarily the case that increased sleep directly leads to increased growth, or vice versa.

Athletes, who put their bodies under a lot of stress and physical pressure, spend proportionately more time in slow-wave sleep than the average person, and growing children spend more time in it than older people. Also, hard physical exercise typically causes a moderate rise in deep slow-wave sleep the next night, all of which supports the cell restoration and repair theory.

The metabolic activity during sleep is mainly anabolic (during which new molecules are constructed and built up) rather than catabolic (where molecules are broken down for subsequent re-use). This also supports the idea that growth and restoration of tissues occur during sleep. The theory has the added advantage of being apparently universal across the animal kingdom, even down to simple unicellular life.

Sleep and body maintenance 

It certainly seems intuitively logical that the body’s quiescence during slow-wave sleep is a good opportunity for the body to focus on physical healing and damage repair. The more active brain wave patterns during REM sleep suggest a restorative function more focused within the brain. Most of our homeostatic processes (the maintenance, stabilization and regulation of the body’s internal environment) occur automatically, and many of them take place during sleep. This is when the pressures and stresses of everyday life are reduced and there is time and opportunity to devote to this kind of stabilization and maintenance work.

On the other hand, whole-body protein synthesis actually decreases during sleep, and it is still not completely clear that more repair work actually occurs during sleep than during waking hours. Also, while the physical benefits of stage 3 non-REM sleep are readily apparent, it is not at all clear what value stage 2 non-REM sleep – which accounts for over 50% of the adult human sleep period.

Theory #3: Brain Plasticity, Learning, and Memory Processing

We all know how important sleep is to our cognitive function. When you don’t get enough sleep, you’re likely to experience deficits in learning, memory, focus, creativity, and more. Because sleep plays an important role in our cognitive function, researchers have theorized that we sleep for the purpose of enhanced brain plasticity (the organization and structure of the brain).

Sleep and memory processing 

Sleep appears to be related to the complex functions of memory and learning in several ways. It has been speculated that it is more efficient for new neuronal connections and pathways in the brain to be cemented during sleep, while there are relatively few external stimuli and little or no new information to process.

Sleep deprivation has been shown to lead to reduced attention and short-term or working memory. This in turn influences what gets saved as long-term episodic memories, but it also impacts the performance of higher-level cognitive functions such as decision-making and reasoning. It should come as no surprise, then, that accidents and industrial injuries tend to increase with sleep deprivation. Interestingly, sleep has been shown in experiments to play a major role, not only in memory consolidation after learning, but also in preparing the memory for encoding before a learning experience.

Several studies have shown how sleep facilitates long-term memory processing, both the conversion of short-term memories into long-term ones, and also the reconsolidating of existing long-term memories. Recent indications are that the REM sleep late in the sleep period particularly benefits procedural memory (the memory of how to do things), while the earlier slow-wave sleep benefits declarative memory (memory of facts and events) more. 

There is also evidence to suggest that REM sleep and dreams play a role in “weeding out” or “pruning back” memories, deleting redundant or unnecessary synaptic connections in the brain, so that some of the less important experiences with which we are inundated over the course of the day are discarded, while the more important memories are retained and consolidated.

Sleep and learning 

Motor learning seems to depend more on the amount of lighter stages of non-REM sleep, while certain types of visual learning are more dependent on both deep slow-wave sleep and REM sleep. REM sleep has been shown to particularly facilitate the retention of emotionally-laden (as opposed to more emotionally-neutral) information.

Neuronal and synaptic activity in the brain during sleep has been shown to be significantly greater in the same areas where learning took place during the day, and the content of most dreams tends to revolve around recent day-to-day events, all of which is consistent with a “replaying” of events during dreams and reconsolidating of the memories and learning recently acquired. Study after study has shown that tasks learned during one day are performed better the next day after a good night’s sleep, and significantly better than after a night of no sleep at all.

Interestingly, species of fish that tend to school (and so, it is argued, have little need for complex, higher-level information processing) are the very species of fish that appear to exhibit notably little, or even no, sleep, which supports to some extent the learning facilitation theory. However, it does not seem to follow that the animals with the largest and most complex brains also sleep the most, suggesting that memory and learning consolidation cannot be the sole reason for the evolution of sleep in the animal world.

Impacts of sleep deprivation on mood

As well as its effects on memory, though, sleep deprivation also has a detrimental effect on mood and emotion, including increased instances of rage, fear or depression, although the exact neural and cellular mechanisms for this are still poorly understood. Alterations in mood also appear to affect our ability to acquire new information and to subsequently remember it. Indeed, some researchers believe that sleep exists principally in order to allow us to dream, which is an essential part of the process of resolving emotional problems (see the section on Dreams).

Some studies have suggested that sleep, particularly REM sleep, can facilitate creativity, flexible reasoning and higher level “insights” (sudden gains of understanding or explicit knowledge). Dreams have long been thought to be instrumental in artistic creativity and idea-forming, although largely based on anecdotal accounts. However, it should be noted that other studies have muddied the waters somewhat, and one study has even positively associated creative children with insomnia and disturbed sleep.

Theory #4: Preservation and Protection

A more anthropological and adaptive theory holds that sleep improves an animal’s likelihood of survival. Specifically, it states that those animals with sleeping habits appropriate to their environment are most likely to survive. It is based on the idea that a sleep period ensures that animals are safely and quietly hidden away at the very time of the day when they would otherwise be most at risk from predators.

Although this theory might explain a period of quiescence or inactivity during times of danger, it does not explain why sleep should leave us so vulnerable and defenseless (with greatly decreased sensitivity to external stimuli, and sometimes complete paralysis) at such a critical time. Intuitively, it would seem that remaining conscious would make us better able to deal with any threats or emergencies. It also doesn’t explain why carnivores like lions, which have few predators to fear, sleep such long hours.

Also, we now know that sleep is not just a passive removal from the environment. Indeed, it appears to be an actual drive in most species (animals will alter their behaviors in order to make sure they obtain sufficient sleep) and the preservation and protection theory fails to deal with this kind of behavior. It is possible that, while such a theory may have been valid long in the evolutionary past, sleep in higher animals has evolved over time to such an extent that it no longer fulfills the same functions.

Theory #5: Early Brain Development

Many studies have demonstrated that sleep is particularly important to the health and development of babies and young children. Animal studies have shown that sleep dramatically enhances changes in brain connections during the period of early development. For example, in experiments with young cats that had just experienced an environmental challenge, animals that were allowed to sleep for six hours after the stimulation developed twice the amount of change in brain plasticity compared to cats kept awake afterward.

This theory is born out by the fact that young children spend much longer sleeping than older children and adults. Babies and infants, who are acquiring information at a rate faster than at any other point during life, sleep the most. Newborn babies can sleep for anything up to 18 hours a day, and 12 hours or more is the norm for toddlers and youngsters all the way up to school age.

REM sleep in particular appears to be important for the development of the brain, especially in the young developing infant. Babies spend most of their time sleeping, and up to 80% of that time may be spent in REM sleep, whereas the older a person gets the smaller the proportion of REM sleep becomes. REM deprivation in infants has been shown to lead to developmental abnormalities later in life. It has been suggested that muscle atonia (the paralysis of the muscles during REM sleep) allows for the formation and activation of synaptic connections in the brain during this time without any potentially dangerous motor consequences.

However, as a counter-argument to this theory, many aquatic mammals (such as dolphins, whales, etc) experience little or no REM sleep in infancy, and the proportion of REM sleep in these animals actually increases as they age.

Final Thoughts

As it stands, scientists do not have one definitive answer as to why we sleep, but that doesn’t mean that we haven’t learned a bunch through sleep research about what sleep does for our health and wellbeing. Sleep affects seemingly every aspect of our health, and as such, it’s important to do what you can to get the right amount of sleep every night.

When we accumulate sleep debt, we function less effectively, feel tired and irritable, make more mistakes, are less creative and, if taken to extremes, ultimately shorten our life span. In the same way as a feeling of hunger reminds us of the basic human need to eat, a feeling of sleepiness reminds us of our essential need to sleep.

Exactly how or why sleep is essential, though, is more difficult to pin down. As William Dement, co-discoverer of REM sleep and pioneering sleep researcher, puts it: “As far as I know, the only reason we need to sleep that is really really solid is because we get sleepy.”