Philosophers and psychologists have long believed dreams point to subconscious thoughts, fears and desires, but new research suggests they have a much more practical purpose, The (Sydney) Daily Telegraph reported Thursday.

Professor Drew Dawson from the Centre For Sleep Research at the University of South Australia said dreaming could be a vital mental tool for sorting and filing information and discarding what is not needed.

"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," he said.

"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 think "dream sleep," also known as rapid eye movement (REM) sleep, is the deepest stage of sleep.

Dawson said 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.

"Your brainwaves while you're dreaming actually look very similar to your brainwaves when you are awake," Dawson said.

That has led some scientists to believe dreaming is an evolutionary survival mechanism, designed to keep our brains alert.

"It might prevent you getting so deeply asleep that you would be vulnerable if you needed to wake up quickly," Dawson said.

Brainwave tests on animals showed that all mammals dream, but whether dogs and cats and other species actually have visual dreams is impossible to know.

"It's reasonable to think they probably do, but we don't know for sure," Dawson said.

Sleep and fatigue expert Dr. Stuart Baulk said humans were essentially paralyzed when they entered dream sleep, with muscle tone changing to prevent the body moving around.

On rare occasions, some people experienced "sleep paralysis," where they woke and were fully conscious but still paralyzed and seeing dreamlike images.

"Although you see the room around you like normal, you might still get the images from a dream," Baulk said.