Why do we sleep?

It’s an age-old question, but one which we’ve all pondered at one time or another: Why do we sleep? Sleep isn’t something that humans do; almost every animal also get some kind of sleep as well. So, why is it such a crucial part of our lives? And why is it so important that we get enough sleep?

What are the stages of sleep?

If you’re unfamiliar with how sleep works, there are essentially three types of sleep. There’s light, deep and REM sleep. Each of the three kinds are crucial, and essential physiological functions take place in each. If you get a healthy night’s sleep, mostly uninterrupted, then you will undergo each of the three phases, and there is no wasted time in any of the stages. However, it does vary between individuals on how much of each stage is required for the best night’s sleep possible. For instance, most middle-aged and older adults would benefit more from receiving more deep sleep and less light sleep. This is because, as most of us get older, we begin losing deep sleep, and it replaced by light sleep.

One way to consider this is to imagine your body having two systems. In order for you to be in deep sleep, they must both be asleep.

Whereas elder people often struggle to get enough deep sleep, youngsters and children will find it far easier to drift off into a state of deep sleep. We’ve all heard the phrase, “sleeping like a baby”. This comes from the stage in which the baby is in deep sleep. This particular stage is where it’s most optimum for the body to grow and renew itself. It’s not just babies who benefit from this phase of sleep, however. Adults also receive a boost in growth hormones during the first deep sleep period of the night.

Many people are unfamiliar with the various stages of sleep. Most of us crave the deep sleep period of the night, even if we’re not necessarily aware that that is what we’re craving. We all have felt the need for a decent night’s sleep, where we’re so fast asleep that we’re almost oblivious to the world around us. We also know the feeling of waking up after what seemed like a full-night’s sleep, only to feel like we haven’t slept at all. However, the mornings where we wake up feeling refreshed are nights where we have had a lot of deep sleep.

Even though the phenomenon of sleeping far predates humans, scientists are still yet to figure out exactly why we sleep. All we know is that we need sleep, but are still unsure as to why this is the case. There are many theories, although we’re yet to settle and agree on a single one. What’s especially interesting to consider is that most humans could survive longer without food than without sleep. Although we don’t understand why we sleep, we’re still able to study and gather lots of information on the functions of sleep, and specifically what happens whilst we’re asleep.

Why scientists believe we need to sleep

Let’s take a look at some of the reasons why scientists think that sleep is necessary to us.

Firstly, whilst asleep, the brain has a lot of time to reorder its inputs, which it’s unable to do while we’re awake. The brain has the opportunity to exercise vital neuronal connections which may otherwise deteriorate with a lack of activity. Think of this function as computer being able to refresh itself.

It’s almost impossible to comprehend just how much data our brains notice, collect and interpret each day. There’s far more data and activity going on within the brain than we’re aware of. Whilst we’re asleep, this gives the brain a chance to process and reorganise some of the the data that it’s collected, and help solve some of problems. This is one of the reasons why we’re often able to make much clearer decisions on things in the morning than we were the previous night. During sleep, it’s believed by scientists that the brain is able to organise and archive memories.

Proper sleep is a time for some real rest, far more than we could get from just sitting or taking it a bit easier through the day. Whilst we’re sleeping, our metabolic rate and energy consumption levels are reduced dramatically. The allostatic load on the body takes a toll, and sleep is a welcomed breather for the body.

During sleep, the cardiovascular system is also able to take a much needed rest. Scientists have discovered that during sleep, people with normal or high blood pressure can see a 20-30% drip in blood pressure, and a 10-20% reduction in heart rate.

Throughout the sleeping period, our bodies are able to create and replace necessary chemicals within the body, repair muscles and tissues, and replace aging or dead cells far more effectively than it’s able to do during the day. Far more growth hormones are released during sleep than they are whilst we’re awake.

Although scientists don’t know why we sleep, they are sure that there’s more than just one single reason for it. It’s believed that there are multiple benefits and purposes of sleeping.

One of the primary reasons for sleep, it’s believed, is that our bodies are able to undergo a rapid rate of recovery. It’s a state where the body is able to rest and repair itself, promote cell growth, and generally have a time period of housekeeping for the body.

Another reason for sleep is protection. This may be easier for us to imagine for animals, but it would have originally being a vital role for ourselves too. Sleeping at night would mean that we’d be much quieter and therefore would be at much less risk of attack by predators.

It might be difficult to imagine in our almost 24/7 modern culture, but there would have been a time upon until recently where the energy regulation of sleep would have played a vital role for us. When food was scarce, it would be necessary for the body to have periods of the day or night where we slept, and therefore used far less energy.

Another crucial reason for sleeping is that it promotes memory consolidation. This means that, as we’re sleeping, our brains can process information which can be used to organise and evaluate memories, and help with long-term learning.

Although these are all assumed to combine to provide reasons for why we sleep, we’re still relatively unsure about each of the reasons listed above, when it comes to explaining them in more detail than simply using them as theories for why we sleep.

Caltech professor David Prober suggests four hypotheses for why we sleep.

Firstly, general life causes damage to the cells. Sleeping offers a time where the body can focus on repairing some of the damage caused to the cells. Researchers find that smaller animals, whose metabolisms are much higher, spend most of the day asleep. Large herbivores, whose metabolisms are slower, subsequently spend much less time asleep.

Another reason, Prober believes, which is vital for us, is rest. Whilst we’re asleep, it provides the body with an opportunity to get a long period of intense rest. While we’re sleeping, we’re able to replenish our body’s energy reserves.

Sleeping allows us to assess and reorganise our memories. Any errors in our brains can be cleared, and that’s one of the reasons why we feel so refreshed and often more optimistic when we wake up after a period of deep sleep.

Prober believes that sleep allows us to process memory and promote learning. Our brains are able to process the information we gather through the day and use it for long-term learning.

Of all the possible functions of sleeping, it’s clear that NREM and REM have entirely different roles, as they are such varying forms of sleep. Scientists studying the brain waves, or the EEG readings, find that they are totally different from one another. Whereas REM sleep waves are very similar to waking brain waves, other than some minor variations, our skeletal muscles are paralyzed throughout REM, though during NREM, sleepers are still able to move around. We have found that memory consolidation and growth hormone releases happen the most during periods of NREM. Our most vivid and complex dreams happen during REM. Finally, thermoregulation takes place during NREM, whereas it does not occur during REM.

What tests are there to measure quality of sleep?

There are four common tests which are used to analyse and measure the effects of stimulants, and also symptoms of particular sleep disorders.

Multiple sleep latency test (MSLT)

The first is the Multiple sleep latency test (MSLT), which measures how long it takes to get to sleep.

Maintenance of Wakefulness Test (MWT)

The second is Maintenance of Wakefulness Test (MWT), which also measures how long it takes to get to sleep.

Wilkinson addition test

The third is the Wilkinson addition test, which measure our cognitive behaviour.

Digital symbol substitution test

Finally, the Digital symbol substitution test also measures our cognitives behaviours.

Many of us will feel as though we’ve been fully recharged and refreshed after a full night’s sleep, feeling like we’ve had our mental fuel tank recharged. However, scientists are still unsure as to why exactly this happens at a biochemical level. We know that despite resting, the brain still actually uses a lot of energy during sleep, so it’s not likely that sleep is only used to rest muscles and recharge energy reserves.

We do know that sleeping is about cycles. As you may be aware, our bodies go through stage after stage whilst sleeping, all in cycles. Firstly, our waking state, or stage 0, transitions in the NREM stage, also known as stage 1. Afterwards, we enter stage 2, and then stage 3, of REM sleep (stage R). The brain may wake briefly after REM, and then go directly back to stage 1 once again. Usually, each cycle will last around 90 minutes. In the later sleep cycles, deep stage 3 may disappear, once the brain has recovered from its need to enter deep sleep.

We can understand sleep by borrowing an analogy from system theory, which demonstrates that sleep is an emergent property of populations of local neural networks which are undergoing state transitions. By looking at it like this, we understand that sleep is an emergent property of some of the brain’s neural networks. This analogy looks t sleep by describing at a complex system which has come from simpler interactions of small elements. We know that a lot of the properties which are found within organismic and evolutionary biology are of this emergent property, and that this explanation shows us why many similar phenomena, including swarming behaviour of insects and birds, and even the patterns of stock markets, occurs.

This is how we’re able to describe whether a person is asleep or awake; they’re two different, identifiable states, even when there is still so much activity going on within the brain at night. We can observe whether somebody is asleep or awake, and falling asleep is the stage at which it is considered o be a shift for the network.

Anatomists have discovered cortical columns in the brain, which are otherwise known as neuronal assemblies. These are believed to be one of the fundamental processing units of our brains, and sometimes adjust from from one state to another altered state, as demonstrated by an input-output relationship. Anatomists have evidence to suggest that the state which is considered to be asleep happens whilst the column generates a larger response to a particular stimulus. The evidence for this is due to the fact that whilst an animal is sleeping, most of the brain’s cortical columns are in the state, and the opposite is true whilst the animal is awake.

As mentioned previously, we all know what it’s like to wake up feeling refreshed after a good night’s sleep. However, we’re all equally as familiar as to how it feels on a morning when we wake up feeling groggy and as tired as we did the previous night, or in some cases, even moreso. One of the reasons for this maybe due to the behaviour of the brain being an emergent property when enough of the cortical columns are still in the sleep state. It’s theorised that the columns interact with one another, and are able to synchronise their positionings via chemical and electrical signals. The chemicals have a tendency to flip between either being in the sleeping position or being in the awake position. However, not all columns follow order, and there is new evidence to suggest that different parts of the brain behave separately from each other throughout sleeping.

Other newly discovered evidence suggests that sleeping allows our lymphatic system to remove metabolic products from the brain and the surrounding tissue, again showing us how crucial sleeping is for our cranial maintenance.

When it comes to the symptoms and signs of our behaviours while we’re sleeping, there is a clear difference between the two. We’re able to observe signs via technical instruments. Signs include things such as blood pressure and reaction times – we are able to actively observe these. On the other hand, symptoms are only observable and experienced by the patient themselves.

For instance, pain is a symptom. Another is sleepiness. Whereas these may be actually taking place, they do not necessarily indicate the need to diagnose a disorder. We all feel sleepy or certain pains from time to time. Even the healthiest athletes will feel more tired than usual, without feeling as though they have reason to. It’s just part of our everyday lives. What separates the everyday sleepiness from being a common occurance we all share to being a legitimate disorder is down to its frequency and intensity levels. We know that people with a sleeping disorder may feel a much stronger need to sleep than those without it. Sleepiness can also be a symptom of another non-sleeping illness, or a reaction to medication or other stimulants. Having a strong urge to sleep doesn’t necessarily mean the patient has a sleep disorder.

Unfortunately, sleep deprivation can commonly cause injuries. It’s been proven that people of all ages are far more likely to suffer an injury if they’re tired. Industrial and car-related accidents are all increased when people are deprived from sleep. We’re all aware of the adverts and campaigns which promote the awareness of driving whilst tired, and how dangerous it can be. Studies have shown that chronic insomniacs are up to 4.5 times more likely to have an accident than others. Older people with sleep deprivation are more likely to experience falls and bone breaks or fractures as well.

Millions of pounds worth of car damage, and worse, physical injury or death, occurs as the result of people driving whilst tired. Slower reaction times are to blame for most of this. In other instances, people fall asleep at the wheel. It’s similar for people who use industrial and potentially dangerous tools and equipment.

Since our younger days at school, we’ve all been told how vital sleep is to our education and learning. A well-rested brain can take in and process far more information than one which is tired and unrested. But, it seems that many of us, mostly adults, ignore a lot of what we already know and don’t get as much sleep as we should. So, what can we do to re-educate people on the health benefits of getting better sleep?

The question should be of particular importance and interest to public health officials, parents, and teachers. And even the general adult should be made aware of the benefits of getting better sleep, and the disadvantages a lack of sleep can bring.

There are a number of free online resources which people can study. Harvard Medical School has its own website, understandingsleep.org – which also includes videos. People who are interested in finding out more about sleep behaviour should take a look at the site.

There are also a plethora of books available, which highlight and tackle some of the problems relating to sleeping. These can be found at your local bookstore, online, or even as downloadable ebooks.

Going further than this, people who have problems with sleeping, or sleep disorders may decide to undergo cognitive behavioural therapy, which has been proven to be one of the best long-term solutions for disorders such as insomnia. However, it requires a lot of instruction and the patient must put in a lot of effort to learn about the practice. However, the benefits are usually universally rewarded with a better ability to sleep and overcome particular disorders, at least to some degree. Some experts believe that cognitive behavioural therapy can be up to 40-50% effective when dealing with insomnia.

A simpler trick which can help you get a better night’s sleep is to improve personal hygiene. Keeping yourself and your bedding clean will help you get a much better night’s sleep.


For the general public to better understand the importance of sleeping, and how to achieve a better night’s sleep, much should be done from the perspective of governments and health agencies and officials. There needs to be more effort, time and money spent addressing the issue, which can offer countless benefits, from increased productivity through to improved general health and wellbeing.