Why cannot people understand their dreams when it is their own brain that creates them?

One theory is that dreams are linked to the way the brain processes memory - though why my dreams usually feature minutely-detailed images of totally fictitious places it's made up on the spot, I cannot imagine! Or perhaps imagination -whatever that is neurologically - can have a high old time without the awake you getting in its way.

Usually we forget dreams on waking, though we might recall bits of particularly vivid dreams because we seem to want to do so. Generally though they mean nothing. They are just jumbled fragments the mind is busy sorting while other areas of the brain are keeping the body alive and you fast asleep so you can't distract it from its important work.

One aspect of the latter duty is simple and obvious when you think about it: it raises the thresholds of your senses so you can sleep without leaving you defenceless. You won't notice ordinary sounds, low light or contact with the bed even when turning over in your sleep; but a sudden loud noise may awaken you, your sight may respond to a sudden rise in light level, someone may wake you by shaking or patting you.

Sleep is a very important function common to most animals - and the brain is pretty busy during it.