Scientists may have finally uncovered why we dream - it could be a crucial tool for sorting and filing information and discarding mental trash, says an Australian researcher.

He added: "If we give someone a complex new task to learn and we let them sleep but don't let them dream, they're almost as bad the next day as if they didn't sleep at all."
Many people mistakenly believe "dream sleep", or rapid eye movement (REM) sleep, is the deepest stage of sleep.
According to Prof Dawson, humans were more deeply asleep prior to dreams. The average sleeper went through several sleep cycles a night, each lasting about two hours and finishing with REM.
Prof Dawson said: "Your brainwaves while you're dreaming actually look very similar to your brainwaves when you are awake."
This has led some scientists to theorise that dreaming is an evolutionary survival mechanism, which helps keep our brain alert.
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Brainwave tests on animals demonstrate all mammals dream, however, whether dogs and cats and other species actually have visual dreams is impossible to know.
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Sleep and fatigue expert Dr Stuart Baulk believes humans were essentially paralysed when they entered dream sleep, with muscle tone changing to prevent the body moving around.
Occasionally, some people experienced "sleep paralysis", where they woke and were fully conscious but still paralysed and seeing dreamlike images.
Dr Baulk said: "Although you see the room around you like normal, you might still get the images from a dream."
Source-ANI