Taking a power nap for an hour during the day can be just as effective as having a whole night's sleep, a new study has suggested.

Scientists found that volunteers who dozed off for 60 to 90 minutes improved their performances in visual learning tasks just as much as if they had slept for eight hours.

They did, however, find that the power-nappers would only show such an improvement if they had dreams during their short rest.

Slow-wave sleep (SWS) and rapid-eye movement (REM) sleep are characterised by different brainwave patterns.

Most dreaming occurs during REM sleep, in which a sleeping person's eyes flick back and forth rapidly.

The psychologists at Harvard University in Cambridge, Massachusetts, tested the volunteers with a visual learning task which involved identifying the position of bars on a screen.

The tests were carried out at 9am, at 7pm on the same day, and 9am the following morning.

Volunteers who were not allowed to sleep saw their performance decline by the evening of the first day.

But those who napped for an hour with both SWS and REM sleep did significantly better at 7pm than those who had not nodded off.

Nappers who never entered REM sleep showed no improvement.

Presenting their results in the journal Nature Neuroscience, the scientists, led by Dr Sara Mednick, concluded: "From the perspective of behavioural improvement, a nap is as good as a night of sleep for learning on this perceptual task."

The researchers also found that napping added to the benefits of a night's sleep.

This emerged after volunteers were tested once at 9am on the first day and then 24 hours later.

Those who took a 90-minute nap on day one were 50% better at remembering the task 24 hours later than those who had not taken a nap.

"Indeed, 24 hour improvement in the nap group was as great as that previously reported after two nights of sleep," said the researchers.

More studies will be needed to generally determine how beneficial the effects of napping are.