photo of a woman sleeping

Every night, billions of people climb into bed, close their eyes and leave the day behind them. Even the animal kingdom joins us in this mass exodus from consciousness.

In the grand scheme of things, our lives are fleetingly brief. So why do we spend around a third of it closed for repairs?

You might be surprised to hear that scientists are still trying to unravel the mysteries of sleep. But they do have a good idea of some of the important functions it serves.

You won’t find total consensus among sleep experts. However, there are three broad theories that receive a lot of support:

3 key theories about sleep:

  • The brain organizes and processes information – a kind of mental housekeeping.
  • The body can recover from the day’s activities and build energy ready for the next day.
  • It’s an important time to help fight illness, recover from injury, grow and develop.

In this article, I’ll be taking a look at these and other theories about the purpose of sleep. I’ll also cover 10 clear benefits that have been shown in research studies, from fewer accidents to less colds and even a better love life!

Memory consolidation: the brain doing housework

image representing brain activity

In a study in 2000, the researchers Terrence Sejnowski and Alain Destexhe offered evidence that sleep is an opportunity to consolidate memories.

This is when short-term memories gained from activities during the daytime are converted into long-term memory.

The researchers suggest that this process and the necessary ‘network reorganization’ takes time and brain power. Thus they need to occur when other awake processes, such as sensory processing, aren’t going on.

In May 2016, scientists from the Harvard Medical School conducted some interesting experiments with mice.

Using special probes, they were able to stop the mice from entering the REM phase of sleep, but without waking them up.

They found that this had a significant impact on their memory of previous days activities, compared to the mice whose REM sleep phase wasn’t interfered with.

In an interview with the BBC, Dr. Williams said:

Disrupting the activity only during REM sleep, and not other sleep, basically obliterates consolidation and memory formation.

However, other researchers have suggested that memory consolidation alone doesn’t justify the enormous amount of time we spend in bed.

Flushing out the brain’s chemical waste

In 2013, researchers from the University of Rochester came up with an alternative theory, also based on research with mice.

They found that cerebral spinal fluid is pumped around the brain at a much higher rate when mice sleep.

They believe this is because the space between brain cells is greater during sleep. This gives more room for the spinal fluid to flow around, gathering up the garbage generated during the day.

They argue that similar actions occur in human brains. They believe that this flushing action was designed to remove the waste chemicals that are produced as part of our brain cell’s natural activity.

Dr. Maiken Nedergaard described this to the BBC as:

You can think of it like having a house party. You can either entertain the guests or clean up the house, but you can’t really do both at the same time.

Recovery and energy building

When we’re awake and active, the body is described as being in a catabolic process. The body releases large amounts of activity-based hormones, which help the body and mind deal with daily demands.

For example, the hormones include adrenaline (the fight or flight hormone), as well as noradrenaline and cortisol (two stress hormones).

When we sleep, the body changes to an anabolic process and the amounts of those hormones are reduced significantly.

Instead, other hormones are released which stimulate the important cellular replication and repair process involved in growth.

One key player is the aptly named growth hormone. Bodybuilders and other sports people are also often told it’s while you sleep that the most muscle growth and repair happens.

It also helps explain why young people need more sleep as they grow and develop. Although not all scientists agree, the majority believe that sleep is a recovery period for the body as well as the brain.

Helps fight illness

photo of a sick woman lying down

We know that our bodies have a very smart immune system which tries it’s best to keep us free from illness.

In simple terms, when the body detects a foreign body, such as a bacterial infection, it releases specific antibodies to attack the unwanted intruder.

This immune system also detects if any of the body’s normal cells have mutated or become cancerous. Again, your immune system will attempt to destroy these cells to keep you healthy.

Researchers have found that without enough sleep, the body produces less cytokines, a protein that targets infection and inflammation. It’s also during sleep that more of the protein is released, helping keep you well.

And in 2019, researchers in Germany found that sleep enhances the effectiveness of T cells, which are immune cells.

Study co-author Luciana Besedovsky reported that:

Our findings show that sleep has the potential to enhance the efficiency of T cell responses, which is especially relevant in light of the high prevalence of sleep disorders and conditions characterized by impaired sleep, such as depression, chronic stress, aging, and shift work.

10 benefits of sleep

1. Improves your mood

photo of a man and woman smiling

Do difficult events or situations seem much harder to cope with when you’re tired? Do you become irrational, more upset or react in a negative way?

Researchers have found that a lack of sleep can lead to more negative emotions. And negative emotions, upsetting events and subsequent stress can result in insomnia. This can result in a vicious circle: poor sleep = negative mood/stress = even poorer sleep.

And in 2018, researchers reported evidence that poor sleep leads to increased negative thinking. Subjects in the study spent longer dwelling on negative thoughts than people who had slept well, who in turn were able to move on quicker from negative thinking.

2. Better mental ability and reactions

Even familiar tasks can seem much more difficult following a bad night’s sleep, and our ability to think straight can be reduced.

Numerous studies have demonstrated that a lack of sleep can also reduce reaction time. For example, studies of college athletes in Iran and army cadet officers in the USA.

Both of these were very fit groups with well-honed reactions to start with. Interestingly, physical strength in the athletes wasn’t affected by losing a night’s sleep. However, both groups showed significant reductions in their reaction times.

3. Less mistakes and accidents

photo of a doctor taking notes

In 2004, medical researchers compared the performance of junior doctors working in intensive care units.

One of the groups worked 24 hour shifts every 3rd night. On average this group made 22% more serious medical mistakes than the group whose working hours didn’t exceed 63 hours per week.

It’s no surprise then that medical establishments around the western world are limiting the number of hours junior doctors can work.

It’s also been found that people are more likely to have a dangerous or even fatal accident at work when lacking sleep. A study of 47,860 people in Sweden over 20 years found that people who reported disturbed sleep were more likely to die in an accident at work.

4. Drive better

Studies have shown that driving while feeling sleepy can be as dangerous as driving after drinking alcohol. You’re also at a higher risk of having an accident.

The AAA in the USA conducted a large review of accidents. They estimated that a sleepy driver was involved in:

  • 7% of all crashes in which a car had to be towed
  • 13.1% of crashes that required someone to be admitted to hospital
  • 16.5% of fatal crashes

And researchers in 2018 found that sleep apnea or insufficient sleep duration were associated with motor vehicle crashes. Sleeping 6 hours per night was associated with a 33% increased crash risk than 7 or 8 hours.

5. Better sex life

A 2015 study at the University of Michigan followed 171 women over a two week period and compared sleep characteristics with sexual activity.

One result showed that just a 1 hour increase in sleep length resulted in a 14% increase in the odds of having sexual activity with a partner. Overall, they concluded:

Obtaining sufficient sleep is important to the promotion of healthy sexual desire and genital response, as well as the likelihood of engaging in partnered sexual activity.

Other studies have shown that lack of sleep lowers both male and female libido and can lead to problems such as erectile dysfunction.

One factor is that good sleep increases testosterone levels, which is a key hormone in boosting sexual drive for both men and women.

6. Stay healthy and fight illness

There are an increasing number of studies demonstrating the relationship between sleep and good health. Some examples include:


Researchers from the Boston University School of medicine studied the relationship between sleep and diabetes in 1486 people.

They found that those who sleep for just 6 hours are 1.7 times more likely to develop diabetes than people who sleep for 7-8 hours. And this figure increases to 2.5 times for those who mange just 5 hours sleep a night.

Weight gain

A study of 500 people over 13 years found that those who slept for less than 7 hours a night on a regular basis were 7.5 times more likely to be overweight.

Part of the problem may be due to our bodies craving more high fat food if we feel tired.

Migraine & headaches

Good sleep may help people who suffer from migraines and headaches. It’s not always clear if poor sleep results in headaches, or if they cause poor sleep though.

In a study in 2013, researchers concluded:

poor sleep quality is uniquely associated with episodic migraine.

They went on to recommend that doctors should also investigate any sleep disorders for patients suffering from regular migraine attacks.

Heart problems

A study in the Netherlands of 20,432 people aged between 20 and 65 looked at the health of those sleeping less than 6 hours. The researchers found that:

  • They were 15% more likely to suffer from cardiovascular disease.
  • 23% more likely to suffer from coronary heart disease.
  • Poor sleep quality carried an even greater risk of heart problems.

Colds and infections

Researchers at Carnegie Mellon University ran a study of 153 adults to examine the relationship between sleep and the common cold.

They monitored their sleep patterns over 14 days. Then exposed them to a rhinovirus, and checked them to see if they developed a clinical cold.

People who averaged less than 7 hours sleep over the 2 week period before infection were 2.94 times more likely to develop a cold than those who slept for longer than 7 hours.

7. Build muscle better

Essential growth and repair hormones are released when you sleep. For that reason, fitness experts emphasize the importance of good quality sleep.

A lot of the medical research looks at this aspect from the opposite end of the telescope.

Here they say that the lack of sleep inhibits the vital repair of muscle caused by injury or training. In other words, your muscle mass and strength can deteriorate over time if you sleep badly.

8. Improve your learning

One of the functions of sleep is to consolidate short-term memories. So all the new information you learn during the day needs sleep to consolidate it into long-term memory.

Moreover, in addition to storing information, sleep may be important in learning new skills.

Research has also suggested that teenagers achieve better outcomes when their sleep cycle is adjusted. Starting school or college later in the day can help ensure they get enough sleep.

9. Less visits to the doctor

People who suffer from very poor sleep, or a significant sleep disorder, have been found to be significantly more likely to consult a doctor.

In some cases, of course, there’s also an underlying health problem which requires medical attention. If you suffer from certain sleep disorders, such as apnea, it’s important to consult a doctor when needed.

But, as mentioned above, lack of sleep can increase significantly the risk of developing other health problems.

So working hard to ensure you sleep well should help keep you away from the doctor. It might also reduce your expenditure on sleep aids and other medication.

10. Reduce pain

A lack of sleep appears to make people more sensitive to pain and/or less able to tolerate pain.

For a long time, scientists have suggested that there is a relationship between sleep and pain, though the mechanism has been elusive.

In 2019, researchers found evidence that sleep deprivation increases pain reactivity in the human primary somatosensory cortex. They found that the temperature threshold where pain was reported was widened by poor sleep.

Interestingly, they suggest that there is a relationship between just one night of worse sleep and perceived pain levels the next day.

They say:

…our findings highlight sleep as a novel therapeutic target for pain management within and outside the clinic, including circumstances where sleep is frequently short yet pain is abundant (e.g., the hospital setting).


The answer to the question of why we sleep could be quite simple. It’s essential for us to be at our best, both physically and mentally.

Modern scanning techniques provide interesting insights into what actually happens to our brain and body whilst we sleep. I suspect we’ll learn even more from the scientific community over the next few years.

And it may also be that it’s just important to help us relax and recover from the stresses and strains of daily life. After all, being alive is an incredible, but not always easy, experience!

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