Curling up for an afternoon nap can improve the brain's ability to learn by clearing out cluttered memory space, psychologists say.

People who nodded off for an hour after lunch performed better in learning tests than those who stayed awake all afternoon, the scientists found.

A study of students revealed that their brains were refreshed by napping only if they entered what is called stage 2 non-REM sleep, which takes place between deep sleep and the dream state, known as REM or rapid eye movement sleep.

The findings support a habit made famous by Sir Winston Churchill, who considered it part of his daily routine to climb into bed at some point between lunch and dinner.

The research follows a recent study by the same group that showed that staying up all night reduced students' ability to cram new facts by nearly 40%, a consequence, they said, of brain regions effectively shutting down through sleep deprivation.

"Sleep not only rights the wrong of prolonged wakefulness but, at a neurocognitive level, it moves you beyond where you were before you took at nap," said Matthew Walker at the University of California, Berkeley.

The findings may explain why our ability to learn falls as we age, since people tend to sleep less as they get older.

Walker's team recruited 39 students for the study and divided them into two groups. At midday, all of the volunteers took part in a learning test designed to exercise a region of the brain called the hippocampus, which is involved in storing memories.

Two hours later, one of the groups settled down for a siesta while the other group stayed awake. Later that afternoon, at 6pm, both groups took part in a second round of learning tests.

Those who napped for an hour not only performed better than the group that stayed awake, they scored better than they did in the first round of tests.

The findings suggest that sleep clears the brain's short term memory and makes room for new facts to be remembered, Walker told the American Association for the Advancement of Science meeting in San Diego.

Previous studies have established that fact-based memories are stored temporarily in the hippocampus before they are moved to the prefrontal cortex region of the brain. "It's as though the email inbox in your hippocampus is full and, until you sleep and clear out those fact emails, you're not going to receive any more mail. It's just going to bounce until you sleep and move it into another folder," Walker said.

In the latest study, Walker's team used electrodes to take EEGs, or electroencephalograms, of the students and identified the sleep phase that was critical for their memories to be refreshed.

Humans spend half of their sleeping hours in stage 2 non-REM sleep. "I can't imagine Mother Nature would have us spend 50 per cent of the night going from one sleep stage to another for no reason," Walker said.