Researchers at the University of California, San Diego report that when it comes to creative problem solving, sleeping on it really does do the trick — provided rapid eye movement (REM) sleep is what you get. Nappers who were allowed to reach the REM stage, which occurs after about an hour of sleep, performed significantly better on certain problems that they had encountered earlier in the day than their non-napping counterparts.

The hippocampus is essentially shut down during REM sleep ... so new information can flow freely and new associations can be created ...

"We found that — for creative problems that you've already been working on — the passage of time is enough to find solutions," said Sara Mednick, in a university news release. "However, for new problems, only REM sleep enhances creativity."

Mednick and her team asked participants to solve several types of problems, testing them once in the morning and once in the afternoon. Between test sessions, participants napped and were allowed to enter into REM sleep, napped and were not allowed to enter REM, or were only given a quiet rest period with no nap.

On one kind of test — the Remote Associates Test (or RAT) — the REM nappers performed about 40% better than they had in the morning. This improvement was not seen in the other two groups. The RAT test probes creative problem solving prowess because it asks the test taker to make associations between groups of words: for example, "cookie, heart, and sixteen" the word that connects them would be "sweet."

On other kinds of memory tests, there were no differences among the three groups of participants.

Mednick and her colleagues suggest that new associative networks are being formed during REM sleep: "Specifically, with REM sleep, there seems to be information flow between an area called the hippocampus, which is very important for learning or (episodic) memories of our own experiences ... and the neocortex, which is more for the associative processes," says Mednick. The hippocampus is essentially shut down during REM sleep, explains Mednick, so new information can flow freely within the neocortex and new associations can be created without the presence of specific memory.

Mednick says that she herself, also a song-writer, has personally tested the current findings: having only a vague idea of the lyrics the wanted a new song to include, she took a 90 minute nap to try to work out this "problem." Says Mednick, "When I woke up, in fact I had the song ready." She also says that she was happy to have experienced what many have claimed anecdotally for a long time, and what her own research now backs up with evidence.

The study was published in the June 8 issue of Proceedings of the National Academy of the Sciences.