But dreams can also occur during so-called ‘slow-wave sleep’. And, although scientists have been studying the dream cycle for decades, no one really knows for sure why we have them.
There are many theories, including that dreams are meaningless and are a simple by-product of neural processes. Some scientists, however, believe dreaming is like a virtual-reality simulation in which we can rehearse threatening situations. Supporting evidence comes from the fact that the incidence of scary situations in dreams (some studies say 70 per cent) is much higher than in real life. Amazingly, dreams also seem able to influence physiological state – one study found that subjects who were deprived of water before they slept, and who drank in their dreams, felt less thirsty when they woke up.