good night's sleep

A new discovery could make it possible to take a "power nap" at the flick of a switch.

Scientists have found a way to turn on deep sleep at will using a machine that magnetically stimulates the brain.

A device worn on the head could in squeeze the benefit of eight hours' sleep into just two or three hours.

Scientists in the US used a technique called transcranial magnetic stimulation (TMS) to induce slow waves - indicative of the deepest phase of sleep and essential for learning ability and mood, in a group of sleeping volunteers.

A TMS device sends harmless magnetic signals through the scalp and skull and into the brain, where it activates electrical impulses.

The researchers found that positioning the TMS machine the right way triggered slow waves that travelled throughout the brain.

Slow wave activity occupies 80% of sleeping hours.

During slow wave sleep, waves of electrical impulses wash across the brain at a rate of roughly one a second.

With each magnetic pulse, the volunteers' brains immediately generated slow waves typical of deep sleep.

"Creating slow waves on demand could some day lead to treatments for insomnia," said study leader Prof Giulio Tononi, from the University of Wisconsin-Madison.

"Theoretically, it could also lead to a magnetically stimulated `power nap' which might confer the benefit of eight hours' sleep in just a few hours."

Prof Tononi believes sleep is essential to prevent the brain overloading.

Memory involves strengthening synapses - connections between brain cells formed by learning.

Sleep might allow the connections created during the day to relax at night, according to Prof Tononi.

The research appeared in an early edition of the American journal Proceedings of the National Academy of Sciences.