It's a mystery why humans and other animals sleep. Leading theories suggest it has to do with memory consolidation or other to benefit other physiological or neural functions. But a new theory holds that sleep is merely a good way to increase an animal's efficiency, by hunting only when the hunting is good, for example, and minimize its risk: The risk of an accident goes down when you're sleeping.

The slumbering mind might not seem like an apt tool for any critical thinking, but humans can actually solve problems while asleep, researchers say. Not only that, but one purpose for dreaming itself may be to help us find solutions to puzzles that plague us during waking hours.

Dreams are highly visual and often illogical in nature, which makes them ripe for the type of "out-of-the-box" thinking that some problem-solving requires, said Deirdre Barrett, a psychologist at Harvard University.

Barrett's theory on dreaming, which she discussed at the Association for Psychological Science meeting here last month, boils down to this: Dreaming is really just thinking, but in a slightly different state from when our eyes are open. [Why we dream is just one mystery of the mind.]

"Whatever the state we're put in, we're still working on the same problems," Barrett said. Although dreams might have initially evolved for a different purpose, they likely have been refined over time so they can serve double-duty: help the brain reboot itself and problem-solve.

Dreams and evolution

A theory to explain dreams, or any human behavior for that matter, needs to take into account evolution, Barrett said. But many early theories of dreaming either didn't address evolution at all, or downright contradicted it, she said.

For instance, Sigmund Freud proposed dreams exist to fulfill our wishes. But such gratification in an imaginary world would do little to help us adapt our instincts to the physical world, which is one key point of evolution, Barrett said.

Others have proposed dreams are more of a side effect of the sleep cycle. Dreams usually occur during Rapid Eye Movement, or REM, sleep. This stage is thought to serve several functions: to rest a part of the brain (since some areas are active while others aren't) and to replenish brain chemicals, such as neurotransmitters.

This has led some to say that dreams happen simply because REM sleep happens, Barrett said. The psychologist Steven Pinker once likened dreams to computer screen savers, saying that it perhaps
"doesn't really matter what the content is as long as certain parts of the brain are active."

However, Barrett disagrees. "My opinion is that, evolution just isn't wasteful, that when things evolve for one purpose, that generally they don't continue throughout time to have only that purpose, but anything else that may be useful about them gets refined," she said in a telephone interview with LiveScience prior to the convention.

In one experiment, Barrett had college students pick a homework problem to try to solve in a dream. The problems weren't rocket science; they were fairly easy questions that the student simply hadn't gotten around to solving yet. Students focused on the problem each night before they went to bed. At the end of a week, about half the students had dreamed about the problem and about a quarter had a dream that contained the answer, Barrett said.

So at least in the cases where problems are relatively easy, some people can solve them in their sleep.