Why Do We Sleep?
Modern Theories of Sleep

Why do we sleep? You have done it every day since you were born - but have you ever stopped to properly ponder this question?

Scientists are yet to establish a unified theory of sleep, mainly because sleep really is a dark area of research. In fact, it wasn't until the 20th century that scientists discovered the human brain is highly active during sleep. Before that it was dismissed as a meaningless area of research.

In this article I will outline some basic facts about sleep which will come in useful in your quest for lucid dreaming.

Sleep Cycles

In the human brain (in fact - in all mammals and birds) sleep has two phases:

Each phase has its own physiological (body), neurological (brain) and psychological (mind) features. They flow together to form sleep cycles.

In the late 1960s two scientists, Rechtschaffen and Kales, defined a typical sleep cycle, displaying varying levels of consciousness. It looks like this:

A complete cycle lasts around 90 to 110 minutes. So eight hours of sleep will give you around five of these sleep cycles per night.

Notice how we are in NREM sleep for the most part. REM sleep only lasts for about 10 minutes at the end of the cycle. However, as the night goes on, some NREM cycles get shorter and REM sleep increases.

Dreaming is more common in REM sleep, although it does also occur during NREM sleep. Now take a look at the features of a normal sleep cycle.

Circadian Rhythms

Your sleep cycles are controlled by the Circadian clock. This is an internal timekeeping device that the human brain uses to control body temperature and release certain chemicals (called neurotransmitters) at the right time.

When your brain releases a neurotransmitter called adenosine at night, you begin to feel sleepy and your body temperature falls. Similarly, in the morning when the Circadian alarm bell rings, your brain releases other neurotransmitters to wake you up.

That is why early risers often find it difficult to sleep in. Their bodies have already been stimulated with wake-up juice. The fact that Circadian rhythms exist tells us that regular sleep is key to our survival.

So Why Do We Sleep?

Having covered sleep cycles, we can move on to the functions of sleep. Why do we sleep - spending a third of our lives in the land of nod?

Sleep is a natural state of rest seen in mammals, birds, reptiles, amphibians and fish. But the variation in the amount of sleep required is stunning:

Bats........................20 hours Lions......................13.5 hours Laboratory rats......13 hours Domestics cats.......12.5 hours Baboons.................9.5 hours Humans..................8 hours Pilot whales............5.5 hours Asian elephants......3 hours Roe deer.................3 hours Giraffes...................2 hours

This has inspired some evolutionary theories of sleep. One theory is that animals who are lower in the food chain sleep less - because they are more vulnerable to attack. However, a conflicting theory suggests that sleep protects us from predators, since we are curled up in a quiet place out of harm's way.

However, both theories have gaps. If we are deprived of sleep, our bodies eventually force us to regain some of that lost sleep - even at the risk of attack by a predator. Why do we sleep at such high risk?

Theories of Sleep

Sleep Theory #1 - To Rest

All day long, we are zipping around in a highly active anabolic state. This is marked by the constant growth and repair of the body's immune and nervous systems, as well as our muscles and bones.

Because this requires so much energy, being awake may only be a temporary state. We use this time to feed and reproduce. That's all. So why do we sleep? Simply: to gain relief from this hyperactive state.

Sleep Theory #2 - To Heal

Sleep also allows us to heal our bodies. In a study on rats, sleep deprivation actually slowed down the healing process of burns. Besides the immune system (our ability to fight disease) sleep deprivation affects our metabolism (our internal chemical reactions). It may also help us save energy for when we most need it.

However, animals in hibernation actually have to catch up on sleep when they emerge. Simply resting in a dark, quiet place does not fulfil their need for sleep if they don't shut down their conscious brain. Something critical happens to the mind during sleep. This may hold the key to psychological reasons why we sleep.

Sleep Theory #3 - To Learn

Sleep may help the human brain get better organized - by filing away important memories and discarding unwanted information.

In a study by Turner et al, 40 people were allowed only 26 minutes sleep per night. They were given cognitive tests which showed their working memory deteriorated by 38% over four days. Without REM sleep, they found it much harder to complete memory tasks and solve problems.

Other sleep experiments have shown that "procedural memory" (your ability to perform certain skills) is dependent on REM sleep. Similarly, "declarative memory" (your knowledge of facts) relies on Slow Wave Sleep.

Sleep Theory #4 - To Dream

Dreaming appears to be a by-product of REM sleep. So is it possible that the the main reason why we sleep is to dream? If so, why do we dream?

The functions of sleep are still unfolding. We understand the natural sleep cycles and the effects of sleep on the human brain and body. However, there is no unifying theory that answers the question "why do we sleep?"

Science has more work to do yet...

Related articles

Sleep Research

History of Sleep

Why Do We sleep?

Sleep Deprivation

Why Do We Dream?

Dream Interpretation

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