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

If you’re getting the amount of sleep that you need for optimal health and cognition, you’re spending roughly 1/3 of your life 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 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?

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 an alert state 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 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 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 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 sleep 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

Theory #1: Energy Conservation

For most of human history, one of our struggles has been finding enough food to survive. 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.

Theory #2: Recovery and Restoration

A common theory that is gaining support is that we sleep to allow our bodies to heal and rejuvenate. 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.

Theory #4: Brain Plasticity, Learning, and Memory

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).


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.


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