"Sleep on it" is standard advice to anyone agonizing over a tricky puzzle. A study of mathematical problem-solving has now shown that a good night's rest really does give you a fresh perspective.

The discovery lends credence to the popular maxim that sleep stimulates lateral thinking, says Jan Born of the University of Lübeck, Germany, who led the project.

Born and his colleagues presented subjects with a series of numbers. They gave participants a simple rule with which to generate a second string of numbers from the first, and asked them to deduce the final digit in this sequence. However, they didn't tell them about a hidden shortcut that allowed the final digit to be calculated almost immediately.

People who tackled the problem in the evening and returned refreshed after eight hours' sleep were more than twice as likely to spot the shortcut as those who had stayed awake. Another group who tried the problem first in the morning, and then spent a normal eight hours of the day awake, were just as bad at spotting the trick as those who had stayed awake all night - so poor performance wasn't simply down to being tired, says Born1.

"This is a good demonstration of something most people know by common sense," says Sidarta Ribeiro, a neurobiologist at Duke University in Durham, North Carolina. The next task will be to work out what the brain gets up to while we sleep, he adds.

Fresh start

During sleep, our brains are thought to reorganize our 'episodic' memories - information about specific places, people, conversations and experiences. This shuffling of the day's events might be responsible for the participants' fresh approach to the puzzle the next day, says Born.

Fresh episodic memories are thought to be stored in a brain area called the hippocampus. But 'permanent' memories seem to be stowed in another region, the neocortex. Born thinks that memories are shunted from one region to another during sleep, and that they are reordered in the process.

Many experts think that rapid-eye-movement (REM) sleep - which accounts for 20-25% of sleep and is the arena in which dreams occur - is the particular type of sleep in which this memory shuffling occurs, says Ribeiro. It could be that participants were dreaming about the puzzle, he points out. This might offer a simple test of whether REM sleep functions in memory reorganization, he suggests - although he concedes that many people forget their dreams as soon as they wake up.

What is certain is that the findings underline the importance of a good night's rest. "People in general do not sleep enough - it's an epidemic," says John Shneerson, a sleep expert at Papworth Hospital in Cambridge, UK. Employers are under pressure to ensure that workers are properly rested, and role models such as British Prime Minister Tony Blair have been criticized for their cavalier attitude to sleep.

A 30-minute 'power nap' would be enough to give workers the benefits of REM sleep, says Ribeiro. Such an allowance could combat stress and boost workplace performance.

"You would expect a power nap to help you be more creative," agrees Born. "But as long as you get around eight hours' sleep at night you should be fine."

References
Wagner, U., Gais, S., Haider, H., Verleger, R. & Born, J. Sleep inspires insight. Nature, 427, 352 - 355, doi:10.1038/nature02223 (2004).

Source