Why do we sleep?

Well the anser to this question has never realy been fully ansered until the 20th century when scientists discovered that the brain is highly active during sleep. It has been found that when we sleep everypart of the brain is alive apart from the logic centre, this goes some way to explaining why our dreams apear so random and provides the reason as to why we so eacyly forget dreams. During REM twise as much blood passes through the brain and a chemical called aseytal coline is sucreated amobalising our movement. We will spend on average 6 years dreaming in our lifetime.

But why do we sleep?
Sleep Cycles.

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

• Rapid Eye Movement a.k.a. REM sleep
• Non-Rapid Eye Movement a.k.a. NREM sleep or Slow Wave Sleep

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. Below.

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. 

Below features of a sleep cycle.

STAGE 1 NREM: LIGHT SLEEP	Your brainwave frequencies descend from relaxed alpha waves (8 to 12 Hz) to dreamy theta waves (4 to 7 Hz). You begin to lose muscle tone, causing twitches and hypnic jerks. You have hypernagonic hallucinations and lose self awareness.
STAGE 2 NREM: LIGHT SLEEP	Your brainwaves show sleep spindles (12 to 16 Hz) and K-complexes. You lose nearly all muscle tone in what is known as sleep paralysis
STAGE 3 NREM: SLOW WAVE SLEEP	Your brainwaves descend to delta waves (0.5 to 4 Hz) - the lowest frequency marking a deep sleep. This is the stage where sleepwalking occurs.
STAGE 4 NREM: SLOW WAVE SLEEP	Your delta waves are more pronounced as you enter deep slow wave sleep. The deepest form of sleep.
REM SLEEP	Your brainwave frequencies jump to highly active beta waves (12 Hz). You have bursts of Rapid Eye Movement and muscle twitches. The most vivid dreams happen during REM sleep.

Circadian Rhythms. (body clock)

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.

A diagram demonstrating an average Circadian Clock.

Functions of Sleep

To find out why we sleep i started to look at various animals and how long they sleep on average. I find these variations fascinating.

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

The above 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?

Sleep theories:

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.

To heal:

Sleep also allows us to heal our bodies. In a study on rats, sleep deprevation 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 concious brain. Something critical happens to the mind during sleep. This may hold the key to psychological reasons why we sleep.

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

"That, if then I had waked after a long sleep, 

will make me sleep again;

and then, in dreaming, the clouds me thought would open and show riches ready to drop upon me; that, when I waked I cried to dream again"

William Shakespeare

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?