The study, published online this week in the journal Memory & Cognition, tested whether sleep or time spent awake worked best in helping people find the solutions to a range of problem solving tasks.
Participants in the study - 27 men and 34 women - were asked to attempt easy and difficult verbal insight problems and, following a period of sleep, time spent wake, or no delay at all, the three groups of participants reattempted previously unsolved problems.
The sleep group solved a greater number of difficult problems than did the other groups, but no difference was found for easy problems.
The authors of the study - Ut Na Sio, Padraic Monaghan and Tom Ormerod all from the Centre for Research in Human Development and Learning at Lancaster's Department of Psychology - concluded that sleep facilitates problem solving but this has its primary effect for harder problems.
Professor Padraic Monaghan said: "We've known for years that sleep has a profound effect on our ability to be creative and find new solutions to problems. Our study shows that this sleep effect is greatest when the problems facing us are difficult. Sleep appears to help us solve problems by accessing information that is remote to the initial problem, that may not be initially brought to mind. Sleep has been proposed to 'spread activation' to the solution that is initially distant from our first attempts at the problem. The advice stemming from this and related research is to leave a problem aside if you're stuck, and get some sleep if it's a really difficult problem."
Previous research has shown that performance on problem solving improves over a period of sleep compared to wakefulness. However, these studies have not determined whether sleep is beneficial for problem solving or whether sleep merely mitigates against interference due to an interruption to solution attempts. Sleep-dependent improvements have been described in terms of spreading-activation, which raises the prediction that an effect of sleep should be greater for problems requiring a broader solution search. We presented participants with a set of remote associates tasks that varied in difficulty as a function of the strength of the stimuli-answer associations. After a period of sleep, wake, or no-delay, participants reattempted previously unsolved problems. The sleep group solved more difficult problems than the other groups, but no difference was found for easy problems. We conclude that sleep facilitates problem solving, most likely via spreading activation, but this has its primary effect for harder problems.