The Purpose of Sleep

Sleep is a normal, indeed essential part of our lives. But if you think about it, it is such an odd thing to do.

At the end of each day we become unconscious and paralysed. Sleep made our ancestors vulnerable to attack from wild animals. So the potential risks of this process, which is universal among mammals and many other groups, must offer some sort of evolutionary advantage.

Sleep is essential for good health. In fact, we need sleep to survive just like we need food and water. So, it’s no wonder we spend about one-third of our lives sleeping.

Many biological processes happen during sleep:

• The brain stores new information and gets rid of toxic waste.

• Nerve cells communicate and reorganize, which supports healthy brain function.

• The body repairs cells, restores energy, and releases molecules like hormones and proteins.

These processes are critical for our overall health. Without them, our bodies can’t function correctly.

An Unanswerable Question? Why do we sleep?

Scientists have explored the question of why we sleep from many different angles. They have examined, for example, what happens when humans or other animals are deprived of sleep. In other studies, they have looked at sleep patterns in a variety of organisms to see if similarities or differences among species might reveal something about sleep's functions. Yet, despite decades of research and many discoveries about other aspects of sleep, the question of why we sleep has been difficult to answer.

The lack of a clear answer to this challenging question does not mean that this research has been a waste of time. In fact, we now know much more about the function of sleep, and scientists have developed several promising theories to explain why we sleep. In light of the evidence they have gathered, it seems likely that no single theory will ever be proven correct. Instead, we may find that sleep is explained by two or more of these explanations. The hope is that by better understanding why we sleep, we will learn to respect sleep's functions more and enjoy the health benefits it affords.

Energy conservation

According to the energy conservation theory, we need sleep to conserve energy. Sleeping allows us to reduce our caloric needs by spending part of our time functioning at a lower metabolism.

This concept is backed by the way our metabolic rate drops during sleep. Research suggests that 8 hours of sleep for human beings can produce a daily energy savings of 35 percent over complete wakefulness.

The energy conservation theory of sleep suggests that a main purpose of sleep is to reduce a person’s energy use during times of the day and night, when it’s inconvenient and less efficient to hunt for food.

Cellular restoration

Another theory, called the restorative theory, says the body needs sleep to restore itself.

The idea is that sleep allows cells to repair and regrow. This is supported by many important processes that happen during sleep, including:

• muscle repair

• protein synthesis

• tissue growth

• hormone release

Brain function

The brain plasticity theory says sleep is required for brain function. Specifically, it allows your neurons, or nerve cells, to reorganize.

When you sleep, your brain’s glymphatic (waste clearance) system clears out waste from the central nervous system. It removes toxic byproducts from your brain, which build up throughout the day. This allows your brain to work well when you wake up.

Research suggests that sleep contributes to memory function by converting short-term memories into long-term memories, as well as by erasing, or forgetting, unneeded information that might otherwise clutter the nervous system.

Sleep affects many aspects of brain function, including:

• learning

• memory

• problem-solving skills

• creativity

• decision making

• focus

• concentration

Emotional well-being

Similarly, sleep is necessary for emotional health. During sleep, brain activity increases in areas that regulate emotion, thereby supporting healthy brain function and emotional stability.

Areas of the brain in which sleep increases activity include:

• amygdala

• striatum

• hippocampus

• insula

• medial prefrontal cortex

One example of how sleep can help regulate emotion occurs in the amygdala. This part of the brain, located in the temporal lobe, is in charge of the fear response. It’s what controls your reaction when you face a perceived threat, like a stressful situation.

When you get enough sleep, the amygdala can respond in a more adaptive way. But if you’re sleep-deprived, the amygdala is more likely to overreact.

Research shows that sleep and mental health are intertwined. On the one hand, sleep disturbances can contribute to the onset and progression of mental health issues, but on the other hand, mental health issues can also contribute to sleep disturbances.

Weight maintenance

Sleep affects your weight by controlling hunger hormones. These hormones include ghrelin, which increases appetite, and leptin, which increases the feeling of being full after eating.

During sleep, ghrelin decreases because you’re using less energy than when you’re awake.

Lack of sleep, however, elevates ghrelin and suppresses leptin. This imbalance makes you hungrier, which may increase the risk of eating more calories and gaining weight. Studies shows that chronic sleep deprivation, even as few as five consecutive nights of short sleep, may be associated with increased risk of:

• obesity

• metabolic syndrome

• type 2 diabetes

Proper insulin function

Insulin is a hormone that helps your cells use glucose, or sugar, for energy. But in insulin resistance, your cells don’t respond properly to insulin. This can lead to high blood glucose levels and, eventually, type 2 diabetes.

Sleep may protect against insulin resistance. It keeps your cells healthy so they can easily take up glucose.

The brain also uses less glucose during sleep, which helps the body regulate overall blood glucose.

Immunity

A healthy and strong immune system depends on sleep. Research shows that sleep deprivation can inhibit the immune response and make the body susceptible to germs.

When you sleep, your body makes cytokines, which are proteins that fight infection and inflammation. It also produces certain antibodies and immune cells. Together, these molecules prevent sickness by destroying harmful germs.

That’s why sleep is so important when you’re sick or stressed. During these times, the body needs even more immune cells and proteins.

How much sleep do you need?

The recommended amount of sleep depends on your age. It also varies from person to person, but the CDC suggests the following durations based on age:

• birth to 3 months: 14 to 17 hours

• 4 to 12 months: 12 to 16 hours per 24 hours, including naps

• 1 to 2 years: 11 to 14 hours per 24 hours, including naps

• 3 to 5 years: 10 to 13 hours per 24 hours, including naps

• 6 to 12 years: 9 to 12 hours

• 13 to 18 years: 8 to 10 hours

• 18 to 60 years: 7 or more hours

• 61 to 64 years: 7 to 9 hours

• 65 years and older: 7 to 8 hours

What happens if you don’t get enough sleep?

Without enough sleep, your body has a hard time functioning properly. Sleep deficiency is linked to chronic health problems affecting the heart, kidneys, blood, brain, and mental health.

Lack of sleep is also associated with an increased risk of injury for both adults and children. Driver drowsiness, for example, can contribute to serious car accidents and even death.

In older adults, poor sleep is associated with an increased risk of falls and broken bones.

Specific consequences of sleep deprivation can include:

• mood changes

• anxiety

• depression

• poor memory

• poor focus and concentration

• poor motor function

• fatigue

• weakened immune system

• weight gain

• high blood pressure

• insulin resistance

• chronic diseases, like diabetes and heart disease

• increased risk of early death