Sleep

Sleep is absolutely vital to the human body. Under normal circumstances, people are active during the day and then sleep at night. In some animals for example, the badger, the pattern is the other way around. Creatures are said to be nocturnal when their main period of activity is during the night.

This variation in activity is know as the "sleep-wakefulness cycle".

Why do people sleep?

This question is often asked and the answer still eludes science. It is known hormone secretion and brain activity varies during the sleep-wakefulness cycle. The notion that sleep is required to restore the body is an old one, but exactly what needs to be restored is still not clear.

Brain wave activity in sleep

A device called the electroencephalogram or EEG. Where electrodes are placed on the scalp can be used to measure and monitor brain activity.

When are person is awake the EEG detects high-frequency (20-50Hz) (but low amplitude) waves . These are known as beta waves and appear to originate from the cerebral cortex.

As a person closes their eyes the beta waves give way to the alpha rhythm. The alpha waves have lower frequency (8 -12 Hz) (but are higher amplitude). And are best seen over the occipital part of the brain (at the back). Alpha waves appear to be driven feedback between the cerebral cortex and the thalamus.

As a person becomes more drowsy and falls asleep the alpha rhythm disappears and is replaced by slower waves (4- 7 Hz) of greater amplitude known as theta waves. Theta waves seem to originate from the hippocampus. Theta waves are interspersed with brief periods of high-frequency activity known as "sleep-spindles".

In very deep sleep the EEG detects waves which are slower still, with a frequency of less than 3 Hz and higher amplitude again. These are known as delta waves and probably originate from activity in the brainstem.

Depth of sleep is judged by how easy it is to arouse someone with a stimulus which can be measured. Sound for instance. Increasing sound levels can correlate to changing brain activities on the EEG. So it is possible to measure the depth of a persons sleep.

Delta wave sleep is sometimes referred to as deep sleep, slow-wave sleep or ortho sleep. It is characterised by slowing of the heart rate, a fall in blood pressure and breathing, relaxed muscles and a drop in body temperature.

After around 90 minutes of delta wave sleep (this varies from person to person) the EEG displays a pattern similar to when someone is awake state with fast beta wave activity. This change in EEG pattern is associated with jerky movements of the eyes (rapid eye movement or REM) , the heart rate and respiratory rate increase.

In the Rapid Eye Movement phase the EEG has the appearance of an awake subject (beta waves) . The REM part of deep sleep is sometimes referred to as paradoxical sleep. During each night slow-wave sleep is interrupted by 4-6 episodes of REM sleep, each of which lasts around 20 minutes.

The amount of sleep a person requires varies over their life time. As we get older we require less sleep. The proportion of REM sleep also declines as we age. If a person is woken up during REM sleep, they are quite likely to talk about a dream. REM sleep is associated with dreaming.

Disrupted sleep and sleep deprivation leads to a person becoming irritable and anxious.
If this is prolonged then severe mental changes can take place, such as feelings of paranoia and being persecuted by others. Quality sleep is essential to the human body.