When you nestle into your warm bed, shut your eyes, and slowly drift off to sleep … do you think your mind and body are shutting down for the night?

Not the case, my friend. While our bodies might remain (relatively) still when we sleep, our minds are hard at work. During these critical hours, a lot of that processing, restoration, and strengthening that’s so important to our bodies and minds is taking place.

It’s crystal clear that we need sleep to live, period. Not only is getting fewer than six hours of sleep on a regular basis bad for your productivity, but it also affects your short- and long-term ability to think clearly. In some cases, sleep deprivation has even been linked to health effects like depression, anxiety, obesity, diabetes, cardiovascular disease, Alzheimer’s, and cancer.

Woof. To help us understand how important sleep is, some scientists have compared sleeping to eating. Sleepiness, like hunger, is a natural, protective mechanism, they say. It’s a powerful message your body sends you to signal that bad things will happen if you don’t hit the hay.

Why Do We Need Sleep?

How come our physical and mental health relies so heavily on those six to eight hours of shut-eye?

While scientists know hunger fulfills our need for nutrients, growth, and tissue repair, it turns out they still don’t know for sure which needs sleep fulfills. To this day, sleep remains one of the great, unsolved mysteries of science.

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But that doesn’t mean those scientists don’t have theories. In fact, there are some really interesting ideas out there for the function(s) that sleeping serves us — some more plausible than others. To get a better understanding of why sleep is so important, let’s dig in to the four most popular theories of why we need sleep.

4 Theories Behind Our Need for Sleep

1) The Inactivity Theory

Inactivity theory, also called “adaptive theory” or “evolutionary theory,” is one of the first theories developed to explain our need for sleep — and it’s also not very widely accepted.

The theory states that there is actually an evolutionary advantage to sleep. Because we’re particularly vulnerable when it’s dark out at night, we adapted the need for sleep as survival mechanism to keep us inactive during those hours of vulnerability.

In other words, animals that were able to stay inactive during these periods of vulnerability had an evolutionary advantage over other animals that stayed active because they were less likely to get killed by predators or to get into accidents while moving around or doing activities in the dark. Sleep was an asset, and through natural selection, we evolved and developed what we now recognize as a need for sleep.

Of course, there are plenty of counterarguments to this theory. For example, isn’t is safer to stay conscious — even if you’re lying awake in the dark — so you can react to a potential threat?

Some scientists pair this first theory with the next one on energy conservation.

2) The Energy Conservation Theory

This is one of the most commonly cited theories on sleep function. As you might guess, this one is all about conserving energy — and it’s especially relevant in times when food is scarce.

The theory asks us to compare two animals:

  1. Animal #1 is active for 24 hours per day.
  2. Animal #2 is active for 16 hours per day and asleep for eight.

The theory states that Animal #2 is more likely to survive during times when food is scarce because they need (and use) less of it for energy. In other words, Animals #2 is trading time for energy: They spend less time awake, and use less energy as a result.

It might be harder for us to imagine this now, living in societies where food sources tend to be readily available. But keep in mind that, relatively speaking, it hasn’t been very long since humans were living in a constant state of food scarcity. In those situations, the competition for — and use of — our energy resources was (and still is) one of the strongest factors in natural selection. It’s a similar concept to hibernation, actually, where animals are inactive and have reduced metabolic rates.

Of course, this theory relies on energy metabolism being a lot lower when we’re asleep than when we’re awake — including when we’re awake, but remaining quiet and still. So, is that true?

According to research out of Harvard Medical School, it is true to some degree: Energy metabolism is reduced during sleep by as much as 10% in humans, and even more in other species. But some researchers argue that the amount of energy we conserve during sleep is relatively small compared to when we’re awake. In that case, it’d be “inconsequential for energy conservation to be considered sleep’s primary function.”

Other research that might contradict this theory shows that brain energy metabolism increases in REM sleep.

3) The Restorative Theories

Alright, so what about restoration and rejuvenation? This theory is one of the most popular, and it actually has several versions. The commonality in these theories is this: Sleep provides an opportunity for our body to repair itself, and to restore things that we spent or lost while we were awake.

What “things” would your body need to restore? This is where the theories branch off into sub-theories. Some have to do with physical restoration, others with mental restoration.

Physical Restoration

The list includes immune function restoration, muscle growth, tissue repair, protein synthesis, and hormone release. When it comes to immune function, for example, our bodies need to maintain a very careful balance of cells and immune responses in order to stay healthy. Studies like this one showed that sleep-deprived animals eventually lost all of their immune function — and died within just a few weeks.

Other studies have found that sleep is crucial for our cellular, organic, and systemic functions. For example, sleep deprivation makes our bodies synthesize protein slower, which can cause us to lose muscle and hinder muscle recovery after damage from exercise, injuries, and so on.

Restoration of Cognitive Function

You may have heard that sleep is good for cognition and memory. Specifically, sleep does two things to help us retain memory:

a) It helps us make new memories by removing us from the constant disruptions we experience when we’re awake. When we’re awake, new situations and stimuli can prevent new memories from consolidating in our minds.

b) It helps us consolidate and prioritize memories according to how important they are to us, and our expectations for remembering them. One study from a German research lab found that sleep helps our memory formation most if you know you will need the information later — like when studying notecards for a test.

This version of the restorative theories is similar to the fourth theory we’ll go over in a second on brain plasticity.

Restoration of Adenosine

Another one of the restorative theories is centered around a brain chemical called adenosine. Our cells release adenosine on a regular basis just by functioning normally. But, unlike chemicals like carbon dioxide, our bodies don’t just get rid of adenosine. Instead, it builds up in our brains throughout the day — and might even contribute to our increasing fatigue as the day goes on.

4) The Brain Plasticity Theory

Finally, we have the brain plasticity theory — one of the more recent theories on why we sleep. “Brain plasticity” is the brain’s ability to change its own structure and organization in response to changes within our bodies and in our environment — and it plays a big role in our ability to learn new information and skills.

Where does the need for sleep fit in? The theory suggests those all-important structural and organizational changes in our brain take place when we’re asleep. Without adequate sleep, we have a hard time learning something new because our brain doesn’t have the opportunity to review and “absorb” the new information.

This is especially true when we’re young. According to research from Harvard Medical School, sleep plays a crucial role in the brain development of infants and young children. You know how infants spend 13–14 hours per day sleeping? If the theory is correct, that’s time spent processing information and creating critical connections in their brains.

Even in adults, lack of sleep can have significant negative affects on the ability to learn, perform tasks, and be productive. For example, one study found that people who went four days with almost no sleep saw their working memory reduced by almost 40% on average.

What Does This Mean for You?

Sleep affects your physical well-being and your ability to learn new skills, absorb new information, and be the productive, happy, functioning person I’m guessing you’d like to be. For most adults, that means getting seven to eight solid hours of sleep every night.

Here’s the good news: If you find yourself in sleep debt, you can make it up. One University of Chicago study followed a group of student volunteers who slept only four hours per night for six consecutive days. All of the changes they experienced during that period of sleep deprivation — from high blood pressure to fewer antibodies to insulin resistance — were reversed when the students made up the hours of sleep they’d lost.

(You can learn more about the science of sleep and the five stages of our sleep cycles by reading this blog post.)

Do you get enough sleep? How does your sleep schedule affect you? Share your experience with us in the comments.

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