Finally, scientists seem to have uncovered the reason behind dreams, which could prove a crucial tool for sorting and filing information and discarding mental trash, says an Australian researcher.
"One theory is that it's a way of allowing your brain to recover and consolidate all the memories and activities of the previous day, like filing time," The Daily Telegraph quoted Prof Drew Dawson from the Centre For Sleep Research at the University of South Australia, as saying.
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.
Prof Dawson said: "It might prevent you getting so deeply asleep that you would be vulnerable if you needed to wake up quickly."
Brainwave tests on animals demonstrate all mammals dream, however, whether dogs and cats and other species actually have visual dreams is impossible to know.
Prof Dawson said: "It's reasonable to think they probably do, but we don't know for sure."
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."