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Why Do We Need Sleep?

Woman happily sleeping

The science of sleep can deepen our understanding of sleep's role in overall health.

It's well known that getting a good night's sleep is as essential for your health as eating a balanced, nutritious diet and exercising regularly. (That's right: over time, not getting enough sleep can lead to many health problems.) But what about sleep makes it so critical for overall wellbeing?

We'll dive into some of the science behind sleep, from theories around why we sleep to actions that your body takes during sleep. Understanding why the body needs sleep—and how sleep can support a healthy lifestyle—is a great way to get inspired to prioritize sleep.

What Is Sleep?

While it might feel like your night of sleep is passive, aside from a dream or two, it's not a uniform activity where your brain shuts off. In fact, it's a pretty complex and dynamic activity. During a good night's sleep, your body is actually experiencing multiple stages of sleep in a cycle. The four stages of sleep are: [1]

  • NREM Stage 1: this stage is basically a transition between being awake and asleep. In fact, if you've woken up during this stage, you may not even be aware of falling asleep.
  • NREM Stage 2: this stage lasts around 20 minutes per cycle and makes up around 50% of your sleep time. During NREM stage 2, your brain consolidates memories, helping to commit information from the brain to long-term memory.
  • NREM Stage 3: also referred to as delta sleep, NREM stage 3 is commonly identified as the deepest sleep stage along with REM sleep. As you progress to this stage of sleep, you'll become less likely to be woken up by environmental noises around you.[2]
  • REM Sleep: the final and deepest stage of sleep is REM sleep. This stage begins around an hour and a half after you initially fall asleep and enter NREM stage 1. During REM sleep, your brain is actually very active, although your body is still relaxed and immobile.[3]

The time spent in each of these stages can change from person to person and night to night, depending on age, sleep patterns, alcohol consumption, and other factors.[4] The body needs to experience the full cycle, and REM sleep in particular, in order to reap the benefits of sleep. 

Scientific Theories About Why We Sleep

Did you know that, on average, about one-third of our lives are spent asleep? No one knows for certain why we sleep, but scientists have developed a few theories to explain why we follow this essential function.[5]

Inactivity Theory

An early theory around sleep was based on the idea that sleeping was a mechanism for survival during periods of vulnerability.[6] Animals that stayed inactive during the night would be less likely to have accidents during activities and/or be killed by predators. Over time, this evolved into modern sleep patterns. However, this theory was considered to be unlikely since being alert is presumably a safer state to respond to predators rather than being unconscious.

Energy Conservation Theory

This sleep theory posits that sleep is a tool to reduce energy expenditure, especially when it's not as efficient to search for food. High competition for resources is a big factor in natural selection, enabling animals to use resources more efficiently and boosting their chances of survival. In humans, research has shown that metabolism is reduced during sleep.[7] For instance, body temperature and caloric demands are much lower during periods of sleep than during wakefulness. This data supports the idea that one of the functions of sleep is to help conserve energy resources.[8]

Restorative Theories

Another well-supported theory for why we sleep is so that the body can repair itself and "restore" balance to activities or losses that occurred during the day. There's a host of evidence that scientists have gathered to support this theory. For instance, while we're awake, our brains produce a chemical called adenosine as a by-product of other activities. A build-up of adenosine can make us feel tired and slow down cognitive function, and as long as we're awake, adenosine will continue to accumulate.[9] When we sleep, the body can clear adenosine from the system and make us feel more alert upon waking up.[10] Additionally, many of the primary restorative functions in the body—muscle growth, tissue repair, protein synthesis, and release of growth hormone—occur during sleep.[11] Finally, some studies have shown that animals will lose immune function without sleep and eventually die.[12]

Brain Plasticity Theory

Research around sleep science is always evolving. One of the most recent theories suggests that brain development is central to our understanding of sleep. "Brain plasticity," or changes to the structure and organization of the brain, is correlated with sleep.[13] Sleep plays a critical role in brain development in infants and young children, who spend a significant portion of their day sleeping and in REM sleep.[14] Scientists also see a link between sleep and brain plasticity in adults based on sleep deprivation's effect on the ability to learn and execute tasks.[15]

3 Things Your Body Does During Sleep
  1. Consolidates memories. When awake, the brain is constantly encoding memories. During REM sleep, these are consolidated and logged into long-term memories as your brain sorts through all of the information from that day.[16]
  2. Releases important hormones. During sleep, the body releases and regulates a number of hormones. For instance, sleep helps maintain leptin and ghrelin levels, which help control appetite and promote satiety.[17] Sleep also helps regulate cortisol levels, an important part of the body's stress response system[18]. During sleep, the body also releases growth hormone, which supports bone and muscle development.[19]
  3. Fights inflammation. While you sleep, your body produces a protein called cytokines, small proteins that can help fight inflammation and infection.[20][21]
How Much Sleep To Get

With sleep being such a critical element of a healthy lifestyle, how can you know how much to get? The amount of sleep you need for your body to experience these important restorative benefits changes by age. Infants need between 14 to 17 hours a day, and teenagers need eight to 10. Most adults need around seven to eight hours of sleep.

However, while the quantity of sleep is important, establishing a consistent sleep schedule might be equally as important. A recent study found that having a variable sleep schedule had a bigger effect on mood and depression than sleep quantity.[22] Establishing a consistent sleep and wake schedule can have a hugely positive impact on your health.

Why is sleep important for the brain?

According to brain plasticity theory, sleep plays a key role in brain development and health. Studies show that during sleep, the brain forms new connections and commits experiences we had during the day to memory. Sleep also allows the body to break down and recycle chemicals released by the brain, such as adenosine, that accumulate while we're awake.

How much sleep do you need?

The amount of sleep you need changes by age. While infants need 14 to 17 hours of sleep, adults typically need between seven and nine hours of sleep. Just as the quantity of sleep matters, having a consistent sleep and wake schedule matters, too—so try to get to bed and wake up at the same time each day.