Lucy, of Sawston Village College, Cambridge, UK, asks :-

Why do we have dreams?

Helen Moss, an experimental psychologist at Cambridge University, responded.

Our dreams can be so vivid, and often bizarre, it seems that they must be telling us something important. Sigmund Freud, the famous psychoanalyst, considered dreams to be "the royal road to the unconscious." He believed that dreams express subconscious mental turmoil and that the understanding of the content of dreams could rid people of inner conflicts.

More recently, however, most psychologists have focused, not so much on interpreting the content of dreams, but on trying to work out what contribution they make to the healthy functioning of our brains and minds. One theory, proposed by the Nobel Prize winner Francis Crick and his colleagues is that we "dream to forget". They suggested that dreams are a way of getting rid of the unwanted mental "rubbish" that would otherwise overload our memory systems. Others have suggested that dreams do not have any clear function but are just the "noise" the brain makes while working overnight. On this view, the brain stem produces pulses of neurotransmitters (the chemicals that pass messages around the brain) which trigger random memories. The brain processes these as if they were real events in the outside world, so producing a flow of sensations. What this theory doesn't tell us, though, is how our brains manage to produce such weird and wonderful stories from these random signals!

What we do know, following the discovery of American sleep researchers, Aserinsky and Kleitman, in the 1950's, is that we do not dream all through the night, but usually only during one type of sleep - rapid eye movement, or REM sleep. The brain activity during REM sleep is very different to that during slow wave sleep and much more like that of a waking brain. We usually have four or five periods of REM sleep during the night, which means we will have up to 146,000 dreams by the time we are 80 years old! And yes, everyone has dreams, even if we do not remember them very well.