photo of a woman sleeping peacefullyEvery night billions of people get into bed, close their eyes and leave the day behind them.

But why exactly do we do spend around a third of our lives asleep?

You might be surprised to hear that scientists are still trying to unravel some of the mysteries of sleep, though they of course have many theories.

In this article, I’ll look at what happens when we sleep, and some of the main theories about why we need it. I’ll also cover why it’s important to get the right amount of sleep, with a look at 10 important benefits of sleep.

What is sleep?

Sleep isn’t simply the opposite of being awake; it’s not a total shutdown of all your systems.

When you sleep you cycle through several different stages, with different things happening in each stage. The diagram below illustrates the various stages of a typical sleep pattern.

diagram showing the different sleep stages

You can see that what happens to your body and brain in each stage is slightly different.

For example, in deep sleep your brain is less active, whereas in REM sleep it can be even more active than when you’re awake.

Another interesting fact is the way your brain waves change when you sleep. Deeper sleep is associated with lower frequency brain waves. And, as illustrated in the diagram below, you experience different levels of consciousness and activity.

Despite all this brain activity, normal sleep is still a physical, mental and emotional resting state. You should become detached from the world and not really be consciously aware.

diagram showing the different stages of sleep and the associated brainwaves


Why do we need to sleep?

Many theories are based on what happens if we don’t get enough sleep. You probably already know that a lack of sleep affects your ability to think properly, and leaves you feeling tired.

After even one night without sleep, concentrating on tasks becomes more difficult and our attention span can be much shorter.

Lack of sleep over long periods can be quite debilitating, as well as having overall health implications.

You won’t find total consensus among scientists. However, the following three theories tend to get the most support:

  • During sleep, the brain can organize and process 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 and recover from injury.

Brain doing housework

image representing brain activityIn 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 occurring.

In May 2016 scientists from the Harvard Medical School reported the results of 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 U.K. 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 spending a third of our lives asleep.

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 postulate 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 UK 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. Here the body releases large amounts of activity-based hormones, which help the body and mind deal with daily demands.

These 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 the above hormones are reduced significantly.

Instead, growth hormones are released which stimulate the all-important cellular replication and repair process.

The growth hormones break down fat to provide energy for repairing tissue and stimulating cell division. These processes are essential to replace old or defective cells. Growth hormones are released in sleep stage 4, which is the deepest part of our sleep cycle.

Bodybuilders and athletes are also often told that it’s while you sleep that the most muscle growth and repair happens.

It also helps to explain why babies and young children 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

a person ill in bedWe 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.

So, for example, when we sleep the body releases a lot more interleukin 1, tumor necrosis factor and natural killer cells.

  • Interleukin 1 is a powerful immune system messenger and supports the body in generating a fever, a sure sign of the body trying to fight an infection. It also helps to reduce inflammation.
  • Tumor necrosis factor helps attack cancerous cells. During sleep, we can have 10 times the amount compared to when awake.
  • Natural killer cells (often referred to as NK cells or K cells) are a type of white blood cell called lymphocyte. They are a fundamental part of the body’s natural immune system, playing an important role in the rejection of both tumors and cells infected with a virus. Unusually, although no immediate reduction is seen if you lose a night’s sleep, there is a 30% reduction the next night.

These factors also help explain why chronic insomnia can have numerous other serious health implications.

Evolutionary adaptation

Another theory is that sleep is an evolutionary adaptation. In the past, we undertook necessary tasks like hunting and inventing wheels during the day.

We then slept during the night when it was safer to hide away from any dangerous animals. Or since it was harder to hunt and gather food in the dark, we slept to conserve energy for daytime activities.

But even this theory is questioned by scientists, who point out that the brain can be just as active and using as much, if not more, energy during REM sleep as it does whilst awake.

The top 10 benefits of sleep

Infographic showing the top 10 benefits of sleep

1) Improves your mood

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?

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 an unpleasant vicious circle: poor sleep = negative mood/stress = even poorer sleep.

Doctors also believe that severe cases of poor sleep can result in mental health issues such as depression.

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 our reaction time. For example, in two studies researchers tested college athletes in Iran and West Point 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

image of someone about to have an accident 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.

You’re also more likely to have a dangerous or even fatal accident at work if you’re lacking sleep. One study found that that:

highly fatigued workers were 70 percent more likely to be involved in accidents than were workers reporting low fatigue levels.

And a large study of 50,000 people in Sweden over 20 years found that workers who reported disturbed sleep were twice as likely to die in an accident at work.

4) Drive better

Studies have shown that in many instances driving whilst feeling sleepy can be as dangerous as driving after drinking alcohol. And you’re definitely 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.

5) Better sex life

a man and woman togetherA 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.

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

There have also been studies to identify why this is the case. 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 large 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

image of a man with a headacheGood 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 2013 study, 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 large 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 3 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 are likely to 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.

Some sleep specialists have even proposed that one of the reasons we dream is to rehearse 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

sleeping pills next to a clockPeople who suffer from very poor sleep, or a significant sleep disorder, are 11% 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 can make you more sensitive to pain and/or less able to tolerate pain. Indeed it can be another vicious cycle of pain keeping you awake, which then leads to feeling more pain.

There are also concerns within the medical profession that a lack of sleep can reduce the effectiveness of pain relief drugs.

Some small scale studies have also suggested that sleep improve a patient’s pain threshold.


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; being alive can be pretty intense at times!

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