HomeSleep ScienceWhy Do We Sleep? The 8 Essential Reasons

Why Do We Sleep? The 8 Essential Reasons

By - Last Updated: July 20th, 2019

It’s a fact we often forget: Almost every animal on the planet needs to sleep.

This, of course, includes us humans.

You spend a large portion of your life asleep. This is in spite of the inherent risk involved with spending long periods of time unconscious and unaware of your surroundings.

Our earliest ancestors took a huge risk when they slept. Not only were they easier targets for predators but sleeping also limited the time they could spend hunting and gathering precious resources.  

The average person sleeps for about 8 hours a night, which works out to one-third of the day. Let’s say you live to be 80. By the time you die, you would have slept for approximately 26.6 years!

You spend a third of your life asleep.

You might wonder why our brain dedicates so much time to sleep. What exactly makes sleep so important that it takes up a third of your life?

It may come as a surprise that sleep doesn’t serve a singular function. Even though it feels like your body is shutting down, sleep is actually an active process that helps you accomplish several important biological functions.

So, what are these functions and how do they affect your health and wellbeing? This graphic breaks them down:

Diagram of body with organs listing what happens when we sleep
The major functions that occur during sleep.

This all leads us to the question:

Why do we need sleep?

Humans need sleep for the following reasons:

  1. Energy Conservation
  2. Brain Waste Clearance
  3. Synaptic Downscaling
  4. Memory Consolidation
  5. Emotional Processing
  6. Body Repair
  7. Hormone Release
  8. Immune System Strengthening

I’ll explain each of these in more detail below.

1. Energy Conservation

You use an incredible amount of energy to keep yourselves alive and stay healthy. All of your energy comes from the breakdown of the food you eat by your metabolism. Any energy that you don’t immediately use gets stored in your liver and muscle as glycogen for later use.

Most people in our modern world have plentiful access to nutrition; however; this wasn’t always the case. Our earliest evolutionary ancestors not only had less access to food but also had to expend more energy to find it.

This meant that our ancestors were burning off more energy than they could store. Humans and animals alike compensated by evolving biological processes that help conserve energy. One of these processes might be sleep.

While many researchers believe energy conservation is a core reason why sleep evolved, it remains a hotly debated theory.

How Does Sleep Conserve Energy?

It’s well understood in science that you use slightly less energy when you’re asleep. When you go to bed, the rate of your energy metabolism reduces by approximately 10%.

Your core body temperature also drops by around 1°F. This may not seem like a lot but you expend a huge amount of energy to maintain your core temperature.

Additional proof comes from comparing the sleep length of different animals. Species with high metabolic rates, such as rodents, sleep much more than animals with low metabolic rate, such as fish.

Despite this evidence, some scientist believes that the small amount of energy conservation we achieve isn’t enough to justify 8 hours of sleep.

The debate continues. We’ll learn more about how sleep affects energy conservation as scientists continue their research.

2. Brain Waste Clearance

While you go about your day, your brain is working hard to make sure everything in your body functions properly. This means your brain has to consume a ton of energy to get the job done. In fact, the brain consumes approximately 20% of all the energy your metabolism creates.

When your brain cells, called neurons, use energy, they create waste byproducts. This metabolic junk builds up in your brain over time and can cause major issues if it’s not removed.

For example, the buildup of certain types of waste proteins called beta-amyloid is thought to be one of the primary reasons humans develop Alzheimer’s disease.

When beta-amyloid sticks together it turns into a toxic plaque that disrupts the communication between neurons. This disruption results in many of the cognitive symptoms, such as memory loss, seen in Alzheimer’s.

Thankfully your brain has found a way to clear out these waste products while you sleep!

Researchers discovered that when you’re asleep your brain cells shrink by up to 60% of their normal volume. This allows the brain’s flushing function, called the glymphatic system, to flow between the cells and wash the waste away.

The glymphatic system is 10 times more active while you sleep than when you’re awake. Unfortunately, this also means that if you consistently get less than 7-9 hours of sleep, or suffer from sleep disorders like insomnia or sleep apnea, you’re not getting optional brain cleansing.

3. Synaptic Downscaling

You’re bombarded with an abundance of new information every day.

In order for you to recall this information, your neurons must undergo activity called synaptic downscaling. This process occurs while you sleep and is termed ‘The synaptic homeostasis hypothesis (SHY)‘ by scientists.

Before synaptic downscaling can be discussed, it’s important to understand how neurons send and encode information.

How Neurons Send Information

Neurons, also called nerve cells, are electrically excitable cells that communicate with other cells by electrical and chemical signalling.

Neurons are essential for receiving sensory information into the brain from the outside world, sending information from the brain to the rest of your body, and sending information from your body to the brain.

Neurons send messages by transmitting signals between one cell to another at an extremely fast pace. This communication occurs at a microscopic area between two neurons called the synapse.

Neurons initially send their message via an electrical signal. When the electrical signal reaches the synapse it can’t pass directly to the second neuron. Instead, it activates the release of chemical messengers, called neurotransmitters.

Once released, the neurotransmitters move across the synaptic gap between the two neurons. When it reaches the second neuron, the neurotransmitters bind into a receptor on the surface of the second neuron, like a lock and key.

The binding on the second neuron’s receptor triggers the signal to be turned back into an electrical impulse and the message can continue on its way.

How Neurons Store Memories

New information such as when you meet someone or learn a math problem is received by your neurons as a unique electrical signal. You can think of this signal as a specific pattern or code that only represents the experience you encountered.

The new signal is then encoded, or learned, by your neurons so it can be recalled later. We refer to this as a memory.

Memories are stored by groups of neurons that are primed to fire electrical signal together in the same pattern that created the original experience. Cells that fire together, wire together.

In order to encode important long term memories, such as your address or date of birth, the strength of the synaptic connections, or the intensity at which these groups of neurons communication must be increased.

This synaptic strengthening processed occurs while you’re awake and continually receiving new information. Unfortunately, this strengthening process can’t go on forever.

Synapses have a limit to how much strengthening they can undergo in one day. When they reach their maximum, they lose the ability to properly send and receive signals. This inhibits learning and memory.

In order to get around this problem, during sleep your brain goes through a process called synaptic downscaling.

Sleep Reduces Synaptic Strength

When you sleep, your brain initiates synaptic downscaling so you can learn new information the next day.

Synaptic downscaling is best achieved during sleep because it’s a period when you don’t experience new learning. Neurons can also take the time to sample this new information in an unbias manner and decided whether it should be stored or discarded.

During synaptic downscaling, brain-wide adjustments are made to reduce synaptic strength where it’s no longer needed. To put it simply, every night the brain takes an audit of the things we learned the previous day. Some of this information is important while other information can be lost.

Let’s say, for example, that you visited a restaurant on your lunch break. When you first walked in, you noticed an abundance of people enjoying their food.

A server wearing a white uniform took your order and brought out your meal. Your food was delicious so you made a mental note to recommend the restaurant to your best friend.

When you go to sleep, your brain replays these experiences and decides which information is important enough to store and which should be removed.

Based on your memorable dining experience, your brain maintains the synaptic strength of the neurons that encode the information about your meal and your intention to tell your friend about the restaurant.

On the other hand, the food other customers ate and the color of the server’s uniform is deemed unimportant. The neurons encoding this unimportant information reduce the strength of their synapses.

How is Synaptic Strength Reduced?

Synaptic downscaling occurs in three different ways:

  1. Reducing the total number of synapses in a neuron group.
  2. Reducing the strength of the electrical signals sent between neurons.
  3. A combination of the above.

The balance between storing and removing brain-wide synaptic strength to a sustainable level is called synaptic homeostasis by scientists.

Synaptic homeostasis during sleep maintains energy, space, supplies, and signal-to-noise ratios. Most importantly, it restores your ability to learn new information and create new memories the following day.

4. Memory Consolidation

You’ve probably already realized that one night of poor sleep is enough to have a major impact on your learning and memory.

Sleep deprivation seriously affects your focus and attention, and this impacts your ability to learn new information. Sleep also plays a major role in storing your long term memories, thus not getting enough inhibits this process.

As I previously described, when your brain acquires new information it goes through the processes of determining which information to keep and which to discard through synaptic homeostasis. Information important enough to keep is stored as a long term memory through another process called consolidation.

The Role of Sleep Memory Consolidation

There are two types of memory that sleep helps to consolidate:

  1. Declarative Memory. Your memory of facts and events. These are memories that can be recalled (or declared), such as your Mother’s birthday or what you ate for breakfast.
  2. Procedural Memory. Your memory of how to do something, such as brushing your teeth or driving a car.

The Sleep Stages and Memory

Scientists believe that both declarative and procedural memory is consolidated during specific stages of your sleep. Sleep stages are divided into two main categories:

  1. Non-Rapid Eye Movement (NREM) sleep is composed of 3 stages, varying from light to deep sleep.
  2. Rapid Eye Movement (REM) sleep is a single-stage when dreaming occurs and eyes move in response to dreaming.
Four stages of sleep NERM and REM sleep with sleeping man

When falling asleep, you first enter stage 1, the lightest stage of NREM. During this time, you feel relaxed and your mind will wander. You’re still semi-aware of your surroundings and are easy to wake.

Your brain will continue to move through NREM until it enters into stage 3, deep sleep, also called Slow Wave Sleep (SWS). During SWS, your brain has very low activity and you’re completely unconscious to the outside world. At this point, you’re extremely difficult to wake.

Once NREM has finished, stage 4, REM sleep begins. During this time your brain is active and you may experience dreaming. You’ll also experience paralysis to keep you from acting out your dreams.

One full sleep cycle takes 90 minutes to complete and will repeat throughout the night. Scientist believes that memory consolidation occurs during stage 3 and 4.

The Memory Consolidation Process

When you’re awake, new information is encoded to an area of the brain essential for learning and memory called the hippocampus.

The hippocampus acts like a layover for long term memory. It can store memories temporarily because its synapses are able to change quickly.

If new information is deemed important enough to be stored as a long term memory, the hippocampus transfers it to another area of your brain called the cortex. Unlike the hippocampus, synapses in the cortex change slowly so it takes time for a new memory to ‘stick’.

During sleep, the hippocampus repeatedly ‘teaches’ the cortex the new information to be consolidated. When you continue to recall or study this information, it strengthens the synaptic connections within the cortex.

Once the synaptic strength of the memory is strong enough in the cortex, it becomes completely independent from the hippocampus. The hippocampus can then discard the information.

As you can see, quality sleep is essential for creating long term memories. This is why you may find it so difficult to learn and recall information when you don’t consistently get the recommended amount of sleep, or suffer from sleep disorders like insomnia or sleep apnea.

5. Emotional Processing

Everyone who has had sleep loss knows that it can make you feel moody, stressed, and irritable. Not only does a lack of shut-eye make you feel crabby but it also increases your risk of developing a mental illness such as anxiety and depression.

But what is it about sleep loss that has such a large influence on your mood? Researchers think that while you sleep, your brain helps you process your emotions, both negative and positive. It’s sort of like having an eight-hour therapy session every single night.

How Does Sleep Process Your Emotions?

Most people agree that dreams, which occur during REM stage, can be very emotional.

Often times there is a link between emotional events that occur during the day with the content of your dreams at night. For example, if you’re experiencing stress at work, your dreams are more likely to be stressful in nature.

It shouldn’t come as a surprise that scientists think you process negative emotions during the REM stage.

When you experience strong emotions such as stress or fear, an area of your brain called the limbic system takes over. A specific structure in the limbic system called the amygdala is highly involved with emotional responses and reaction to stimuli.

Although we don’t completely understand how, scientists believe that the REM stage of sleep is responsible for reducing the reactivity of the amygdala to emotional stimuli, especially negative emotions.

REM sleep also processes emotional memories using similar mechanisms I discussed in the memory consolidation section of this article. During REM sleep, an interaction between the hippocampus and amygdala occur to processes emotionally linked memories we learned the day before.

6. Body Repair

Whether you realize it or not, every single day your body accumulates damage to its DNA, cells, organs, and tissues. Luckily, humans have evolved an amazing repair system that works while you sleep.

Wound Healing

How does sleep help you heal your wounds? Sleep expedites the healing process.

A research study was conducted to evaluate wound healing times of patients that had different amounts of sleep. The patients that slept the most healed the fastest. In fact, their wounds healed an entire day faster than those who had slightly less sleep.

Other studies have shown that sleep deprivation inhibits the part of the immune system that’s responsible for fighting off infection. The reduced immune response caused a significant in delayed wound healing compared to those that had normal sleep.

Tissue Repair

Your normal daily activities place a lot of stress on your cells. In order for your body to continue functioning, your cells need to be repaired or completely replaced depending on the amount of damage they’ve undergone.

While you’re in deep sleep (stage 3) the pituitary gland, a pea-sized organ at the base of your brain, releases large amounts of growth hormone (GH). GH plays a role in many biological processes throughout the body including cell and tissue repair.

Interestingly, scientists have found that competitive athletes spend more time in deep sleep compared to non-athletes. This is probably because they place more stress on their bodies and are more prone to injury, and thus require more tissue repair.

DNA Repair

Not only does sleep help to heal your cells and tissues, but it also plays an important role in repairing your DNA.

Damage to your DNA is a normal occurrence. In fact, it has been estimated that an individual cell can suffer up to one million damaging DNA changes per day. Thankfully you have a large arsenal of DNA repair mechanisms, including sleep.

A recent study discovered that damage that occurred to the DNA in brain cells was repaired faster and more efficiently during sleep than during waking hours. They also found that sleep deprivation resulted in a larger accumulation of DNA damage, which took longer than usual to repair.

7. Hormone Release

Hormones are special chemical messengers that control most major bodily functions. Hormones work by circulating around your body to target specific organs and regulate how they function.

One example is estrogen, the primary female sex hormone.

Hormone release is highly regulated by your circadian rhythm, the 24-hour cycle that regulates many physiological processes. Interestingly circadian rhythm also regulates wake and sleep times.

Many hormones are only released during sleep and may also be tied to specific sleep stages.

As discussed previously in this article, the release of growth hormone increases during deep sleep. Other hormones released during sleep include prolactin and luteinizing hormone.

Sleep loss has a negative impact on our hormone balance. For instance, scientist discovered that sleep deprivation reduces leptin, the hormone that tells your body to feel full once you’ve had enough to eat.

Similarly, gherlin, the hormone that stimulates your appetite, increases after sleep deprivation. This imbalance in leptin and gherlin may explain why sleep loss is associated with obesity.

8. Strengthening the Immune System

Your immune system is made up of a vast network of cells, tissues, and organs that work together to protect your body against infectious disease and cancer.

Just like any other part of your body, the immune system must undergo a certain level of maintenance to properly function. Much of this maintenance occurs during sleep.

Increasing the Immune Response

Scientist discovered that sleep increased the strength and activity of certain immune cells called T cells. Your T cells fight against viruses that infect and live within other cells of the body. Examples of these viruses include the flu, HIV, and herpes.

T cells circulate throughout your body, looking for specific targets indicating that a cell is infected with a virus. When the T cells identify a target, they activate sticky proteins known as integrins that allow them to attach to the infected cell. Once attached, the T cell destroys the infected cell along with the virus.

Researchers found that sleep increases the activity of T cells and makes them better at identifying virus-infected cells.

This might explain why you feel excessively sleepy (hypersomnia) when you’re sick. It’s your body’s way of helping your immune system become more efficient.

Antibody Production

Sleep also plays a role in antibody production. Antibodies are proteins made by your immune system and can identify and bind to other proteins, called antigens.

Antigens are found on cancer cells, bacteria, and viruses. When an antibody binds to an antigen, it sends a signal to the immune system to attack the pathogenic agent.

Scientists have found that sleep loss inhibits the production of new antibodies and reduces antibody responsiveness to infectious agents.

These studies highlight that sleep is absolutely essential to keeping you from getting sick!