Why Do We Sleep? (The 4 Theories Explained)

Why Do We Sleep? (The 4 Theories Explained)

If you’re like me, you might find the mysteries of the universe interesting… or you just get really annoyed when you don’t understand something!

One thing I was wondering recently was why we sleep? Why is it necessary to spend 8 hours per night in an unconcsious state when you could be doing other productive things like watching netflix or knitting!

So I set out to discover a satisfying answer for why we sleep using the latest available knowledge, and put in terms that normal people can understand.

So why do we sleep?

While we know sleep is extremely important – it has been observed in a wide variety of life forms, both animals and plants, and when humans or other species are denied sleep they experience adverse health conditions – the science of sleep is still poorly understood. There are four main, competing hypotheses, all of which are detailed below.

The most compelling and convincing seem to be that sleep is a period of necessary rest and tissue regrowth for the body, and is also a time of organisation in the brain. There is intense brain activity during the different stages of sleep, but again, we know little about what is actually taking place.

Now, that’s barely a complete explanation given the depth of this topic, and you’re going to want to find out what happens when we are denied sleep too.

There’s one extremely interesting case of an American in the 1960s who willingly subjects himself to sleep deprivation (it’s illegal to study it now for ethical reasons), so read on to learn all about that.

Why do we sleep?

If humans didn’t need to sleep, we would have 8 extra hours in the day. That could be time spent, building shelters, hunting or gathering food and sustenance, procreating or other activities that would help our survival and procreation which would lead to the trait of ‘not sleeping’ being selected for. T

he fact that it isn’t implies that sleep fulfills an important purpose, one which at the very least offers us more survival and evolutionary value than being awake all night.

So it is not enough to simply say, ‘it gives us a rest’.

While the exact nature and reason for sleep still eludes us, we have made progress in the understanding of what goes on during sleep and there are 4 major credible theories that hope to answer the questions of ‘why do we sleep?’

1. Inactivity Theory

This theory, one of the earliest that had been suggested, is that sleeping is done in the dark and thus keeps us out of harm’s way in the most dangerous time. We’re unlikely to meet any nocturnal predators or make any falls because we can’t see if we’re fast asleep.

This idea has been discredited since and is easily refuted with the argument that if being inactive was beneficial, why not simply rest while staying awake?

In this instance, it would be far easier to react to any dangers.

2. Energy Conservation Theory

This theory states that it is more beneficial to retain energy than to spend time in the dark being productive, at a time when it’s surely less efficient than the day.

When food is scarce, as it always has been outside of the last 70 years, spending 1/3rd of the time in a resting and inactive state may save precious energy.

It’s also been shown that your body’s metabolism uses 10% less energy when you sleep, which seems to agree with the theory.

3. Restorative Theory (Theories)

The next explanation is that the body uses the time spent asleep to repair and rejuvenate itself, perfoming processes that cannot be perfomed during an active state. Much of the empirical data we have collected supports this hypothesis.

For example, it is known that muscle growth, tissue repair and the release of growth hormones occur during sleep.

4. Brain Plasticity Theory

One of the more recent suggestions has been that sleep plays a role in the structure and organization of the brain.

We do know that brains are where the majority of activity takes place, life forms without any kind of brain like plants are generally devoid of sleep and the human brain waves change depending on what stage of sleep you are in.

We also know that memory is affected by poor sleep, which implies that the brain needs to sleep to organze memories. Overall, this theory is becoming more and more compelling.

How do we know sleep is important?

As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy.

This is what the discoverer of REM sleep and sleep researcher William Dement told National Geographic. The fact is, we don’t know all that much about sleep at the moment. Despite this, we are very clear on how important sleep is to humans and other animals.

An analytical approach to working out whether sleep is essential leads to making the following three statements, all of which we would expect to find if sleep was optional rather than necessary.

1. Animal species that do not sleep at all
2. Animals that do not need recovery sleep after staying awake longer than usual
3. Animals that suffer no serious consequences as a result of lack of sleep

We have found no animals that satisfy any of these criteria with some specific but irrelevant examples of animals that don’t have brains.

As such, this indicates that sleep simply must play an important role in the survival and proliferation of all animal species.

Additional further evidence comes our way by way of study into the effects of sleep deprivation. While there is little data from scientific research into sleep deprivation – forcing people to stay awake for days is not making it past any ethics committees – there is one curious example, Randy Gardner.

In 1964, Randy Gardner broke the record for the longest period of sleeplessness with 11 days and 25 minutes.

The reason why this case study is so valuable is that it was attended by prominent sleep scientist including the aforementioned William Dement, which offers us some scientific validity and analysis.

While in many ways Randy was able to function competently, his condition did show serious cognitive and behavioral changes which ranged from moodiness, problems with concentration and short term memory, paranoia and hallucinations.

In one stirking moment, he was asked to subtract 7 repeatedly starting from 100, he made it to 65 before stopping and stating that he forgot what it was that he was doing.

This is compounded by studies into sleep deprivation on animals, where such strict ethic rulings are not required, where rats that were forced to go without sleep ended up dying between 2 to 3 weeks in.

These negative effects all imply that something happens during sleep that allows us to function normally.

What happens during sleep?

Sleep is not just an on or off switch. When you rest for the night your brain and body go through many different stages where different physiological functions are done. The most obvious example is that of NREM and REM sleep.

– NREM makes up the start of your sleep and is made up of stages N1, N2 and N3. This is where you begin as you fall asleep going sequentially through the stages.

* N1 – This is the ‘falling asleep’ stage where you are very easily woken up, it last about 5 to 10 minutes.
* N2 – This is called ‘light sleep’ and your heart rate slows and your body temperature drops.
* N3 – This is called ‘deep sleep’ where you are difficult to wake up and is considered the most refreshing and restorative period of sleep.

– REM (rapid eye motion) sleep is so called because your eyes dart beck and forth beneath your eyelids and this is the stage where you dream.

Your body goes through sleep cycles that last about 90 minutes and consist of each of the stages of NREM and then REM sleep, with the duration of each stage changing throughout the course of the night.

The reason we understand how sleep is broken up into different stages is through observation and testing.

For example, by examining brain waves we can see that N3 sleep is when the larger, slower waves happen which is why it’s also referred to as ‘Slow Wave Sleep’. We also know from studies that subjects who get N3 sleep feel the most refreshed and relaxed the next day.

However, we still do not know everything about sleep and we also don’t even know what we don’t know!

While we do not know exaclty what happens in every moment of sleep, here are a list of the known physiological functions that occur during sleep that do not occur during awakened periods. I’ve included the stage at which each function occurs at the end.

– Your heart rate slows and body temperature drops. (N2)
– The brain begins bursts of rapid, rhythmic brain activity known as ‘sleep spindles’. (N2)
– Your body repairs and regrows tissue. (N3)
– Your body builds bone and muscle. (N3)
– Your immune system is strengthened. (N3)
– The brain produces slow brains waves known as ‘delta waves’. (N3)
– Your heart rate and breathing quickens. (REM)
– The brain shows intense activity. (REM)
– The body and muscles become relaxed but immobilized. (REM)
– You dream. (REM)

As you can see, there is a lot going on. And much of it we don’t seem to understand. Take the brain waves for example, although we can see that all humans exhibit the same brain waves during their sleep cycles, we can only guess at what’s actually going on.

So when researchers show that after sleep, their subjects retain information better and have imrpoved memory, they can only hypothesize that is something to do with those brain waves, we don’t yet have the apparatus to find out.

Why don’t we know that much about sleep?

It’s sort of embarrassing. It’s obvious why we need to eat, for example, and reproduce … but it’s not clear why we need to sleep at all.

These are the words of Dr Michael Halassa, a neuroscientist at New York University. The reason sleep has been shrouded in mystery for so many years is that, unlike many other biological functions, it is not clear from a human perspective what exactly is going on.

Even something that might seem msyterious to an initialy observer like your heart beating, would take only a little cursory examination to realise the truth.

The first time you observe someone with a major cut that is bleeding and you see the spurts come out at around the same pace as a heart beat will answer your questions. Sleep, however, is quite different.

The first problem is it is difficult to study. There is simply little way of knowing exactly what is going on. Our best research to date has been to monitor the physiological functions, see what’s going on, then make some inferences from that.

This has helped us in some ways discover that it’s probably a time of repair and building tissue.

What we have also discovered is that much of what happens during sleep occurs inside the brain. Different periods of sleep have wildly different brain waves, as shown in the following graphic, but we don’t know exactly what causes those brain waves or what the function of them is.

We also notice that during REM sleep we experience intense brain activity and illogical and seemingly random dreams, but again we don’t know the causes or consequences of those things. As such, our best guesses come from inferences rather than pure observation.

One last small point is that there is little money to be made in examining sleep. Unfortunately, money talks, and this is not an area where there are huge corporoations willing to fund cutting edge research in hope of a lucrative cure for a disease or new product.

What other animals/life need to sleep?

It used to be the case that sleep was something that was only observed in mammalian species that had brains, the theory arising from the fact that sleep was mainly observed and studied in mammals.

The form of rest that would be seen in say, insects for example, was referred to as ‘torpor’ and was thought as distinct from human and mammalian sleep.

This view appears to have become outdated as we have learned more about the nature of sleep in other species.

One of the key findings is that we have observed ‘homeostatic’ sleep, which is the tendency to catch up on sleep when you didn’t get much, in species such as fruit flies.

In fact, it seems that many species suffer the same problems from sleep deprivation as ourselves including a study showing that flies that were sleep deprived has problems with long-term memory!

Our current belief is that a wide range of species exhibit signs of sleep to some degree, including mammals, birds, reptiles, amphibians and some fish and even insects.

As far as we can tell, it appears to be a requirement in all mammals and most other animals. That link is an excellent wikipedia article full of knowledge if you’re interested in this topic, by the way.

One last point on hibernation which is not actually sleep, it is a form of torpor, which is similar to sleep in that it is a rested and unconscious activity but it does not precipitate many of the important physiological processes that sleep does.

In fact, when animals wake up for hibernation, they often find themselves tired and in need of sleep!

What happens when we don’t get enough sleep?

As mentioned above, possibly the best explanation of why we need to sleep is simply that we feel bad when we don’t.

This is a very shallow reason in some ways, it’s hardly the kind of answer that would impress an alien species who was interested in this novel ‘sleep’ concept.

But the amount of negative consequences that have been observed in studies and by health professionals provide an overwhelming response to the idea that sleep might not be so important. Here are some of those consequences.

1. It shortens your life expectancy.
2. Puts you at risk of serious medical conditions including obesity, heart disease.
3. You experience impaired memory.
4. It increases the likelihood of being in a car accident. So called ‘drowsy driving’ accounts for thousands of crashes, injuries and fatalities each year. You’re actually 3 times more likely to be in an accident if you get 6 hours of sleep or less per night.
5. Over time, it can lead to premature wrinkling and dark circles under your eyes, affecting your appearance.
6. It is associated with depression.
7. It is associated with a lower sex drive.
8. Getting less sleep can compromise your immune system, increasing the chances that you get sick.
9. Shortened sleep is associated with higher rates of certain types of cancer, including breast cancer, colorectal cancer and prostate cancer.
10. Brain functions are affected, including alertness, decision making, problem solving and reaction time.

I’ll bet you’re planning to get a full 8 hours tonight after reading that, it’s quite the list! This information was derived from these websites, click the links to read more.

Why do we need less sleep as we age?

Our need for sleep changes as we grow older, here’s a quick guide.

While we don’t fully know what goes on during sleep, it is telling that the younger you are the more sleep you need. This implies that sleep has an important connection with growth and learning, both of which are much more associated with children and teenagers than with the elderly which might explain the drop in the need for sleep.

One interesting example is that of babies, in addition to sleeping for up to 13 hours per day, up to 50% of that time is spent in REM in comparison to 20% for a normal adult.

REM being the sleep with high brain activity and intense dreaming, this supports the hypothesis that this brain activity has something to do with language formation, memory and learning, which would all be very useful to a 6-month old who is exploring the world.

On the other side of the spectrum, the sleep that is needed by older people is less simple to understand and is more complicated by the dangers of adult life.

Alcohol, caffeine and other drugs, a busy working life and the stress of bringing up kids and grandkids can all contribute to a worsened sleep schedule, whether desired or not. The Sleep Foundation has this to say on the subject:

Many older adults, though certainly not all, also report being less satisfied with sleep and more tired during the day. Studies on the sleep habits of older Americans show an increase in the time it takes to fall asleep (sleep latency), an overall decline in REM sleep, and an increase in sleep fragmentation (waking up during the night) with age. The prevalence of sleep disorders also tends to increase with age. However, research suggests that much of the sleep disturbance among the elderly can be attributed to physical and psychiatric illnesses and the medications used to treat them.

That same article states that according to a poll by NSF in 2003, 44% of older persons suffer the symptoms of insomnia at least a few nights or more. If you’re struggling with sleep, I highly recommend checking out this article I wrote on falling asleep faster, it’s the product of a decade of learning of all the different issues that can cause sleep hygiene and is the reason I’ve gone from being the worst sleeper ever to regularly being asleep inside 15 minutes.

Why do we dream?

Dreams, which are essentially hallucinations that occur during sleep, are a curious case. On the surface, they are a bizarre phenomenon. Your brain imagines a scene that is apparently random and where logic is thrown out the window, but it can still affect you emotinoally in the same way to the extent where people regularly stay happy or scared or upset for hours from the experience of a dream they had!

Like a lot of things about sleep, there is not enough concrete evidence to confidently say that we know what’s going on. With dreams, there are a few competing theories.

1. Dreams as therapists. This is where you can play through scenarios in safe environment, reflecting and learning on the emotional issues in your life.

2. Dreams as fight-or-flight training. One of the most active parts of the brain during REM sleep is the amygdala which is the part of the brain associateed with the survival instinct and fight-or-flight response. This theory offers the idea that the brain needs to keep its survival response sharp and does this during dreams and the situations that you are put in.

3. Dreams as your muse. Given your total lack of logic in dreams, they can be a catalyst for creative endeavors or linking connections that your otherwise more logical conscious self would ignore.

4. Dreams as memory aides. As dreams are correlated with intense brain activity, it is thought that they could be important in memory. Consolidated important stuff, getting rid of the stuff you don’t need. And dreaming could be a byproduct of your brain doing this reorganizing.


After researching and writing all that, I must say that I’m more convinced than ever that sleep is a hugely beneficial and necessary part of our lives.

It’s all too easy to stay up late watching Netflix or whatever you’re preferred lazy leisure activity is, but getting good sleep is something that I’m going to be even more motivated to achieve going forward.

If you found this article, then it’s likely you struggle with sleep, whether that’s falling asleep quickly or just generally getting a good night’s sleep. I used to struggle massively, a typical night might take 3 hours of tossing and turning in bed before I lose consciousness.

These days, it’s less than 15 minutes practically every night. If you wanna know how I did it then read this article, it’s a decade of learning about how to improve sleep crammed into 4000 words. Enjoy!

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