Dream on for power

A 'power nap' can be as good for you as a whole night's sleep - but only if you dream, a new study suggests.

Volunteers who dozed off for 60 to 90 minutes improved their performance in tests by as much as those who had an extra eight hours of sleep.

But this happened only when the nap included rapid eye movement - a type of sleep in which eyes flick back and forth behind their lids - which indicates dreaming.

'From the perspective of behavioural improvement, a nap is as good as a night of sleep for learning,' chief scientist Dr Sara Mednick concluded.

Volunteers were given a visual learning task, which involved identifying the position of bars on a screen.

They were tested at 9am, 7pm the same night and 9am the next day. Those allowed to snooze for more than an hour did significantly better than non-nappers in the evening.

But this was only if they were able to enter the REM state from the more common 'slow-wave' sleep.

Volunteers who had a 90-minute shut-eye on day one were 50 per cent better at remembering the task 24 hours later than the non-nappers.

'Indeed, 24-hour improvement in the nap group was as great as that previously reported after two nights of sleep,' wrote Dr Mednick.

The results of the study, by Harvard University in Massachusetts, are published in Nature Neuroscience journal.