A type of dreamy sleep that occurs more frequently in the early morning is important for solving problems that cannot be easily answered during the day, a study has found.
The discovery could explain many anecdotal accounts of famous intellectuals who had wrestled with a problem only to find that they have solved it by the morning after a good night's sleep.
Scientists believe that a form of dreaming slumber called rapid-eye movement (REM) sleep, when the brain becomes relatively active and the eyes flicker from side to side under closed eyelids, plays a crucial role in subconscious problem solving.
Dreams normally occur during REM sleep which occupies about a quarter of the total time spent asleep at night. REM sleep alternates with deep sleep and occurs as four or five bouts of sleep that get progressively longer as the night turns into morning.
In a series of tests on nearly 80 people, the researchers found that REM sleep increases the chances of someone being able to successfully solve a new problem involving creative associations – finding an underlying pattern behind complex information.
The volunteers were asked in a morning session to solve a series of creative problems. They were told to either sit on the problem until the afternoon by either resting and staying awake or by taking a nap monitored by the scientists to see whether it involved any REM sleep – it produces characteristic eye movements and brain-wave patterns.
In the afternoon, they were asked to solve the problems again. Those people who had enjoyed REM sleep improved significantly, by about 40 per cent, while the other volunteers who had not had REM sleep showed little if any improvement, according to the study published in the journal Proceedings of the National Academy of Sciences.
The researchers suggest that it is not merely sleep itself, or the simple passage of time, that is important for the solving of a new problem, but the act of being able to fall into a state of REM sleep where the brain slips into a different kind of neural activity that encourages the formation of new nerve connections.
"We found that for creative problems that you've already been working on, the passage of time is enough to find solutions. However, for new problems, only REM sleep enhances creativity," said Sara Mednick of the University of California San Diego, who led the study.