Most of us have a good idea of what it ‘feels’ like to be asleep. There is a loss of consciousness, reduced movement and perhaps the experience of dreaming. But how does sleep differ from the loss of consciousness associated with general anaesthesia or a coma? Does sleep have a function? Do other animals sleep in the same way and for the same reasons that we do?
As far as we know, sleep happens in all animals that have a nervous system – from worms and flies to birds and humans. It is defined as a state in which there is loss of consciousness, decreased responses to sensory stimuli, a general reduction or inhibition of muscle activity and reduced metabolic activity. In humans, the two main phases of sleep are rapid eye movement (REM) and non-REM (NREM or ‘slow wave’). These phases are characterised by specific patterns of electrical activity in the brain (see Box 1).
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