Understanding Why We Sleep
Jake Comfort | Updated: June 2022
Jake Comfort | Updated: June 2022
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Everyone knows that they feel better after a restful night’s sleep. Everyone also knows how awful it can feel when you don’t get enough sleep or the sleep you get isn’t good quality rest. We can feel how getting or not getting sleep impacts our minds and bodies, but many people don’t understand why we need sleep.
In fact, sleep experts and scientists aren’t fully sure why we need sleep. As a result, there are several different theories on why sleep is necessary for people. We’ll look at some of the most popular sleep theories in the current literature to help you understand exactly why it is we need to sleep.
Different Theories of Sleep
There are several theories of sleep. Each one has its own strengths and weaknesses. Additionally, each theory might be only part of the whole reason we sleep. Scientists differ on these theories, but they all offer a good explanation of why we need to sleep. Some of these theories explain why we need sleep at all. Others explain why most people prefer to sleep at night and be active during the daytime. Moreover, more than one theory can be true at a time. Check out the research we’ve done and let us know what you think the best explanation is for why we sleep.
The Inactivity Theory
The first theory that we’ll cover is called the inactivity theory. It is also known as the Adaptive Inactivity Theory. This theory argues that sleep is a protective mechanism that evolved to help keep humans safe at night. It also argues that sleep is necessary for other animals because they need to be protected during the parts of the day when they’re not active. The theory presumes that the sleep drive evolved in response to vulnerability to predators and the fact that humans don’t see as well in the dark.
One piece of evidence that supports this theory is that humans who were less vulnerable at night have a competitive advantage over others people that were engaging in nighttime activity. This kind of activity creates a greater risk of accidents and increases the risk of being attacked by nighttime predators.
Supports of this theory point out that sleeping at night might have been especially important to protecting children. That’s because children’s brains and bodies aren’t fully developed, so they’re at a greater risk of accident or injury if they perform activities at night. As a result, natural selection could have favored humanity’s ancestors that were more likely to stay inactive and out of danger during the night.
This theory also explains why human sleep tends to last the entire night. This pattern helped ensure humans were more likely to be safe when they were at the greatest risk of danger while allowing for maximum productivity during the day. This was helpful because humans can see better and perform better during the day. As a result, activities undertaken during the day are more likely to be more productive. After all, it’s easier to find and prepare food when you can see what you’re doing.
The inactivity theory isn’t perfect, however. There are several arguments people make which they claim expose holes in the theory’s logic. For example, it is much easier to react to danger and defend yourself or retreat to safety if you’re awake. As a result, this theory doesn’t completely explain why we need sleep or why humans generally prefer to sleep during the night.
The Restorative Theory
The next theory we’ll look at is the restorative theory. This theory argues that sleep evolved the way it did to help the body and brain repair damage and develop muscles and bones. One of the best pieces of research on this theory points out that protein synthesis and tissue repair rates are greater while a person is asleep. As a result, sleep is essential to healing. It is also essential to help proper development for younger bodies.
In addition to sleep being essential for the body to grow and repair itself, it’s also required for your brain to operate at its best. That’s because the brain undergoes its own form of repair while you sleep. Special cells called glial cells clean up toxins in the brain using cerebrospinal fluid. Two of the proteins that are removed in this process are amyloid and tau. These two substances are thought to be related to the onset of Alzheimer’s disease as well as other forms of dementia.
Furthermore, sleep is necessary for your immune system to function at its best. There are lots of studies that show animals lose immune system functions when they’re deprived of sleep. Your immune system also uses sleep to do things like produce and release white blood cells, which are one of the key ways your immune system helps prevent you from getting sick. Sleep also promotes the release of certain substances and hormones which help regulate your immune system, remove toxins from your body, and helps your body fight off viruses and bacteria.
Another supporting element for the Restorative Theory points out that being awake causes cells to produce more of a substance known as Adenosine. This is produced by cell activity and energy consumption. Adenosine causes sleepiness as it increases over the course of the day. The more adenosine your body contains, the more tired you’ll feel. Adenosine declines while you sleep as your body removes it.
In summary, there are a few key points supporting the Restorative Theory. These are:
- Repair of tissue and DNA during sleep
- Muscle growth
- Release of growth hormones
- Release of immune-supporting molecules like cytokines
The Energy Conservation Theory
Another argument for why we need sleep is the Energy Conservation Theory. This theory argues that evolution favored human ancestors who remained inactive at night and during parts of the day. Conservation of energy is essential in a world like the one our ancestors grew up in. Food was less available, so conserving calories and energy stores became key to survival.
Sleep helps conserve energy two different ways. First, it conserves existing stores of energy. That means the food and calories you take in during the day last longer. Additionally, sleep means you need don’t need to consume as much food as you do during the daytime. That’s because metabolism decreases by 10% when you’re asleep. Moreover, your body’s core temperature drops when you’re asleep, which means you need even less energy to run your metabolism.
Storing and saving energy is important. Having the right amount of energy available to the body goes a long way in determining how well you preform in your daily tasks. In the past, this meant ensuring that you had enough energy to gather food and obtain more energy. In the modern age it means that you can execute the duties required by your job and other daily tasks. In a way, it’s not that different to our ancestors. After all, your job and daily activities are the means by which you can afford food so you can continue to live.
Another argument in favor of the Energy Conservation theory is that non-rapid eye movement sleep and body temperature regulation evolved at the same time. Scientists take this as an indication that energy conservation was an important factor in the development of both of these bodily functions.
Some scientists see a similarity between the Energy Conservation theory and the Inactivity theory. They argue that sleep may be a protective mechanism which prevents humans from becoming prey for predators and accidents while also reducing inefficient use of calories.
The Brain Plasticity Theory
One of the latest theories of why we sleep is known as the Brain Plasticity Theory. This theory looks at how the brain changes over the course of a lifetime. The theory goes that brain changes and sleep changes could be related.
There are a few different ways that the brain changes as we age. Some of the most important ones are:
- Strengthening or weakening of synapses
- Individual neurons change as a response to events and aging
- Gray matter proportions can change
- Recovery after stroke or injury
- Some activities move from one part of the brain to another
One of the strongest pieces of evidence for the Brain Plasticity theory of sleep is that sleep has a clear role in the brain’s development over time. Infants sleep the majority of the day – as much as 17 hours. This large amount of sleep is essential for growing bodies and minds to get the support they need. This fact also explains why older people need less sleep. Their bodies and brains aren’t growing like they used to. As a result, adults don’t need as much sleep as children and teens.
Evidence also points to the fact that sleep can help the brain restructure and reorganize. Sleep also helps you learn how to perform cognitive tasks. Studies show that sleep affects the strength and speed of synapses and the way they communicate between neurons. As a result, it can be essential to long-term memory and other essential cognitive processes.
Another thing that we can look at to understand the Brain Plasticity theory of sleep is the effects of sleep deprivation. When people don’t get enough sleep they suffer from cognitive problems. Some of the most common issues include:
- Poor concentration
- Reduced performance on memory tests
- Decreased ability to solve problems
- Reduced executive functions
Other studies show that sleep helps the nervous system reboot. During this time the nervous system checks and reinforces the connections between the brain and the body’s other systems. This makes a lot of sense, as we’ve already discussed how processes like metabolism and body temperature change when you fall asleep. Additionally, the way the immune system and body release certain hormones and chemicals while you sleep helps support this theory as well. In fact, the impact that sleep has on immunity that people who have enough sleep have a stronger immune response to vaccines than people who haven’t had enough sleep.
Understanding Why We Sleep
As you can see, there are several different theories that explain why we sleep. Some of these theories focus on why we sleep at night. Others focus on how sleep interacts with the brain and the body. Each theory offers a valuable observation about why we need to sleep. One thing that all the theories agree on is that getting the right amount of sleep helps people live their best lives.
As a result, it may not matter why we sleep. What’s important is that the human body requires sleep to function at its best. Whether we sleep at night to avoid predators, conserve energy, restore and repair the body and mind, or help with processing memories, sleep is essential to all living things. Stay tuned for more updates as the latest sleep science comes out.
Berger RJ, Phillips NH., Energy conservation and sleep, Behavioral Brain Research, July-Aug. 1995
Grønli J, Soulé J, Bramham CR., Sleep and protein synthesis-dependent synaptic plasticity: impacts of sleep loss and stress, Frontiers in Behavioral Neuroscience, Jan. 21, 2014
Dang-Vu TT, Desseilles M, Peigneux P, Maquet P., A role for sleep in brain plasticity, Pediatric Rehabilitation, April 2006
Joshua E. Brinkman; Sandeep Sharma, Physiology, Sleep, StatPearls Publishing, Jan. 2018
Siegel JM., Sleep viewed as a state of adaptive inactivity, Nature Reviews, Neuroscience, Aug. 5, 2009
Bjorness TE, Greene RW., Adenosine and Sleep, Current Neuropharmacology, Sep. 2009
Anil Kumar, A review on Alzheimer’s disease pathophysiology and its management: an update, Pharmacological Reports, Apr. 2015
Patrick Y, Lee A, Raha O, Pillai K, Gupta S, Sethi S, Mukeshimana F, Gerard L, Moghal MU, Saleh SN, Smith SF, Morrell MJ, Moss J., Effects of sleep deprivation on cognitive and physical performance in university students, Sleep and Biological Rhythms, Apr. 13, 2017
Maurizio Gorgoni, Is Sleep Essential for Neural Plasticity in Humans, and How Does It Affect Motor and Cognitive Recovery? Neural Plasticity, 2013
Markus H. Schmidt, The energy allocation function of sleep: A unifying theory of sleep, torpor, and continuous wakefulness, Neuroscience and Biobehavioral Reviews, Nov. 2014