We spend roughly one-third of our lives sleeping1, 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 trying to uncover the exact role of sleep.
We know that not getting enough hours of sleep is detrimental to our overall health and well-being. Sleep deprivation2 can lead to trouble concentrating and learning, worse mood difficulties, and chronic health issues like heart disease, diabetes, and more. Also, as is the case with rare fatal familial insomnia3, without sleep, we can die.
While we understand the importance of sleep, we don’t have a clear-cut 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?
Every night when you lie down to sleep, your body and brain undergo a variety of changes3 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 four stages of sleep which consists of two types of sleep: non-rapid eye movement (non-REM) sleep and rapid eye movement sleep (REM) sleep.4
Learn more: Non-REM and REM Sleep
The third stage of non-REM sleep is referred to as deep sleep. This sleep stage is central to our ability to recover from exercise and injury. This is followed by a single stage of REM sleep.4
REM sleep is different, with brain waves similar to those that occur when we’re awake. Experts believe both non-REM and REM sleep are important for memory consolidation.4
Each of these non-REM/REM cycles lasts about 90-110 minutes5, and we continue to cycle through each stage while we sleep. To reap the full benefits of a night’s rest, healthy adults should experience a sleep duration of around 7-9 hours6 each night.
Current Scientific Theories on Why Humans Sleep
Over the years, various 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 Theory
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 energy7 and require less food.
This idea that sleep plays a role in conservation is supported by the fact that our metabolism slows by up to 10 percent while we sleep. Another component supporting this theory is that glycogen levels (glycogen is the fuel that our brains use for energy) drop during the day and are replenished during sleep8, 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.
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 decreases9, and so we use less energy to maintain our body temperature, also suggesting that one of the primary functions of sleep may be to conserve energy.
Theory #2: Restorative Theory
Another theory garnering support is that we sleep to allow our bodies to heal and rejuvenate.7 It is now widely thought that sleep has a kind of housekeeping 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.7
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 us to recover and start over the next morning.7
The physical healing of wounds is expedited by sleep, and sleep strengthens the immune system10 in general, with expats adding that sleep deprivation is linked to an increased risk of infection or inflammatory diseases.
Sleep and Growth Hormone Development
Sleep, especially stage 3 slow-wave sleep11, has also been associated with increased levels of growth hormone levels in the body. The human 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 it12 than older people. Also, physical exercise typically causes a rise in slow-wave sleep13.
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.
Sleep and Body Maintenance
It certainly seems intuitively logical that the body’s quiescence during slow-wave sleep is a good opportunity for it 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.
Theory #3: Brain Plasticity Theory
You probably 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).7
Sleep, Memory Processing, and Learning
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 deprivation14 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.14
Researchers hypothesize that REM sleep specifically is important for storing memories15. 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 Early Brain Development
Another component of the Brain Plasticity Theory is sleep’s role in early brain development in babies and children. Young children spend much more time 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.
REM sleep appears to be especially important for brain development16. Babies spend roughly 16-18 hours a day sleeping, with about half of that time in REM sleep17, and the older a person gets the less REM sleep they get. REM deprivation in infants could lead to developmental delays18.
Impacts of Sleep Deprivation on Mood
Sleep deprivation also has a detrimental effect on mood and emotion, including increased negative moods, such as feeling cranky, irritated, or worried.2
This may be something you are aware of already. Think about how you feel after a night without enough sleep. You’re probably more irritable, right? Perhaps things bother you more easily and you feel more stressed out.
Unfortunately, these negative emotions can wind up not only impacting your relationships with other people but stress, in particular, could make it harder for you to fall asleep the next night. Therefore, when you are sleep-deprived, it’s especially important to find ways to improve your mood and manage stress.
Theory #4: Inactivity Theory
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.7
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.
Our 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 through sleep research about what sleep does for our health and well-being. 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, 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.
Nicole has spent much of her life pursuing health and wellness for herself and those around her. For four years, she has used her education in biology and her passion for a healthy lifestyle to craft educational articles and coach hundreds of people in diet and nutrition.
- Aminoff, Michael J., Boller, François., Swaab, Dick F. “We spend about one-third of our life either sleeping or attempting to do so”. Handbook of Clinical Neurology. 2011.
- “What Are Sleep Deprivation and Deficiency?”. National Heart, Lung, and Blood Institute. Last modified March 24, 2022.
- Khan, Zalan., Bollu, Pradeep C. “Fatal Familial Insomnia”. StatPearls. Last modified February 13, 2023.
- “Brain Basics: Understanding Sleep”. National Institute of Neurological Disorders and Stroke. Last modified July 19, 2023.
- Patel, Aakash K., et al. “Physiology, Sleep Stages”. StatPearls. Last modified January 26, 2024.
- Watson MD, Nathaniel F., et al. “Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement of the American Academy of Sleep Medicine and Sleep Research Society”. Journal of Clinical Sleep Medicine. 2015.
- “Why Sleep Matters: Benefits of Sleep”. Harvard Medical School. Last modified October 21, 2021.
- Bellesi, Michele., et al. “Sleep and Wake Affect Glycogen Content and Turnover at Perisynaptic Astrocytic Processes”. Frontiers in Cellular Neuroscience. 2018.
- Szymusiak, Ronald. “Body temperature and sleep”. Handbook of Clinical Neurology. 2018.
- Garbarino, Sergio., et al. “Role of sleep deprivation in immune-related disease risk and outcomes”. Communications Biology. 2021.
- Tarasiuk, A., et al. “Role of growth hormone-releasing hormone in sleep and growth impairments induced by upper airway obstruction in rats”. European Respiratory Journal. 2011.
- Li PhD, Junxin., Vitiello PhD, Michael V., Gooneratne MD, Nalaka. “Sleep in Normal Aging”. National Library of Medicine. 2017.
- Park, Insung., et al. “Exercise improves the quality of slow-wave sleep by increasing slow-wave stability”. Scientific Reports. 2021.
- “Sleep On It: How Snoozing Strengthens Memories”. National Institutes of Health. 2013.
- “REM sleep may help the brain forget”. National Institutes of Health. Webpage accessed February 11, 2024.
- Wolfe, Kathy., Ralls, Frank M. “Rapid eye movement sleep and neuronal development”. Current Opinion in Pulmonary Medicine. 2019.
- Grigg-Damberger MD, Madeleine M., Wolfe MD, Kathy M. “Infants Sleep for Brain”. Journal of Clinical Sleep Medicine. 2017.
- Yasova Barbeau, Daphna., Weiss, Michael D. “Sleep Disturbances in Newborns”. Children. 2017.