Why Do We Sleep?

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What happens when we sleep?I’ll try and keep this light but I find it interesting and perhaps you will, too.

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 (theeye-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 andanother cycle begins.

We go through four or five of these cycles on average per night butwith each 90 minute cycle, the duration of the REM sleep increases

and that of NREM sleep decreases.

Why do we Sleep?

  • (i) Restoration: It is felt that both REM and NREM sleep serve a restorative, replenishing function. NREM restores bodily processes which have deteriorated during the day, while REM sleep is a time for replenishing and renewing brain processes. – Also, children grow and develop physically, while they sleep.

  • (ii) Evolutionary Theory: According to this theory, sleep is an advantage because it keeps the animal immobilized for long periods, being immobile it will be less conspicuous to would-be predators and therefore, this keeps it safe.

Different species sleep for different periods. Those at risk from predators sleep very little while predators that sleep in safe places and can satisfy their food and water needs fairly quickly, sleep for much of the day.

Sleep might also help to prevent exhaustion in which case, sleep has a functional purpose.

Sleep Deprivation

The need for sleep seems so important that total sleep deprivation lasting several nights might be expected to have serious consequences. A number of studies have shown however that the main consistent effects of sleep deprivation are: drowsiness, a desire to sleep and a tendency to fall asleep easily.

When sleep is abruptly reduced (as in the case of hospital doctors who may be on duty for 72 hours at a stretch) the effects can be quite serious i.e. irritability, intellectual inefficiency and an intense fatigue and need for sleep.

If people or cats are deprived of all sleep for a period of days, they will eventually go to sleep standing up. The body seems to have a strong need for REM sleep and if this is deprived, when people eventually fall asleep they go straight into REM sleep as if to catch up on it.


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