Why do we dream?

Every night you spend more than two hours dreaming, in episodes that each last around five to twenty minutes. Yet nobody knows for sure why. Scientists still can't say where in the brain dreams begin, or what they are for.

What we do know is when most of them happen. Dreams come mainly during REM sleep, named for the rapid eye movements that give it away. In REM, your brain is busy, with activity that looks a lot like being awake. Your body, meanwhile, is almost completely paralysed. That is lucky: people whose paralysis fails during REM can physically act out their dreams.

So what are dreams for? There are several serious ideas. One says a dream is your brain telling itself a story: during REM, some regions switch on only in fragments, and the cortex stitches that patchy input into something that makes sense. Another says dreams are rehearsals, a safe place to practise facing threats. A third links dreaming to learning, and a 2010 Harvard study found that dreams were correlated with improved learning.

These ideas can overlap, and the question stays open. Part of the difficulty is that nobody can watch a dream directly. Scientists only get the dreamer's memory of it, told in words after waking, and dream recall is extremely unreliable.

So the next time you wake up from a strange dream, remember that you have just visited one of the everyday experiences science still can't fully explain.