A nap during the day doesn't just beat tiredness, but actually improves the brain's ability to absorb new information, claim US scientists.

Volunteers who slept for 90 minutes during the day did better at cognitive tests than those who were kept awake.

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The latest study, from the University of California at Berkeley, suggests that the brain may need sleep to process short-term memories, creating "space" for new facts to be learned.

In their experiment, 39 healthy adults were given a hard learning task in the morning - with broadly similar results, before half of them were sent for their siesta.

When the tests were repeated, the nappers outperformed those who had carried on without sleep.