Volunteers' sleep state was tested.

But the "power-nap" only works if the sleep is of the right quality, say the experts from Harvard University, US.

And experts say that a full night's sleep is still necessary for many vital body functions, even though a short sleep may boost learning and memory.

Many famous people have claimed that it is possible to get by on just a few hours' sleep a night.

Yachtswoman Ellen MacArthur sailed solo around the world while sleeping only occasionally and for very short periods.

However, there are plenty of others who say they cannot function properly without the full eight hours.

We should not conclude that we can do with just a nap. Dr Derk-jan Dijk, University of Surrey

The Harvard research, published in the journal Nature Neuroscience, compared the learning and memory skills of two groups of people during a single day, and again the following morning.

One group was told not to sleep at all during the day, and, as expected, their performance tailed off into the afternoon and evening.

However, the other volunteers were allowed to have an hour or 90 minutes nap at 2pm.

The researchers tested the brainwaves of the "nappers" to check the quality of their sleep.

Dream state

They were looking for two different sleep phases - slow wave sleep, and rapid eye movement, which is normally associated with dreaming.

Those whose sleep involved both phases fared significantly better than those who had no sleep when given the learning test later in the day.

Volunteers who never reached rapid eye movement sleep did not perform as well - although even this "poor-quality" sleep did prevent some of the deterioration in performance.

Remarkably, over 24 hours, the performance of those who took a good-quality "power-nap" was as good as volunteers in previous studies who were tested after two full nights' sleep.

The researchers wrote: "From the perspective of behavioural improvement, a nap is as good as a night of sleep for learning on this perceptual task."

Dr Derk-jan Dijk, from the Sleep Research Centre at the University of Surrey, said that there was increasing evidence that a combination of "short wave sleep" and REM sleep was important for learning and memory.

However, he added: "We should not conclude that we can do with just a nap.

"Sleep is useful for more things than just these particular tests.