What Happens when We Sleep and What is the Purpose of Sleep?

Sleep and dreaming has fascinated man for centuries. Modern research has helped us understand sleep better. So what happens when we sleep and why does the body need sleep?

Sleep Schedules

Humans share with other mammals a biological clock known as the circadian rhythm that closely follows the night-and-day periodicity of our environment. This cycle tends to have a natural period of 25 hours.

Circadian rhythms cause the jet lag that bothers many air travellers. This difficulty in adapting to the new time cycle often persists for several days. Apparently, it is easier to return to an old cycle than to adjust to a new one.

Depth of Sleep

Some people are readily aroused from sleep. Others are hard to awaken. - This often depends on the stage of sleep:

There are five stages of sleep:

Stage one - The heart-rate begins to slow down, the muscles relax and it is easy to be woken up.

Stage Two - A deeper state of sleep than previously but stillfairly easy to wake someone.

Stage Three - Sleep is becoming deeper. The sleeper is now quite unresponsive to external stimuli and so is difficult to wake. Heart-rate, blood pressure and body temperature all continue to drop.

Stage Four - The sleeper now enters 'delta sleep' (deep or quiet sleep) and will spend up to 30 minutes in this stage. About an hour has elapsed since stage one began. It is difficult to wake the sleeper but something highly personally relevant i.e. a baby crying can rouse even a deep sleeper.

This is not the end. The cycle now reverses:

The sleeper re-enters stage 3, then stage 2, but instead of re-entering stage 1, a different kind of sleep (Active sleep) appears. This is when pulse and respiration rates increase as does blood pressure. The brain is active and yet it is even more difficult to wake someone from this kind of sleep than the deep sleep stage 4.

Another characteristic of active sleep are the rapid eye movements (the eye-balls moving back and forth, up and down, together) under the closed lids (i.e. rapid eye movement or REM sleep).

While the brain is active here, the body is not. REM sleep is characterized by muscular paralysis so that all the tossing and turning and other typical movements associated with sleep in fact occur during stages 1-4 which collectively are called non-rapid-eye-movement sleep. (NREM).

After 15 minutes or so of REM sleep, we re-enter NREM sleep and another cycle begins.

We go through four or five of these cycles

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