CHICAGO - BEING deprived of sleep even for one night makes the brain unstable and prone to sudden shutdowns akin to a power failure - brief lapses that hover between sleep and wakefulness, researchers said on Tuesday.

'It's as though it is both asleep and awake and they are switching between each other very rapidly,' said Dr David Dinges of the University of Pennsylvania School of Medicine, whose study appears in the Journal of Neuroscience.

'Imagine you are sitting in a room watching a movie with the lights on. In a stable brain, the lights stay on all the time. In a sleepy brain, the lights suddenly go off,' Dr Dinges said in a telephone interview.

'This involves more structures changing than we've ever seen before, but changing just during these lapses,' he said.

He and colleagues did brain imaging studies on 24 adults who performed simple tasks involving visual attention when they were well rested and when they had missed a night's sleep.

The researchers used a type of brain imaging known as functional magnetic resonance imaging, or fMRI, which measures blood flow in the brain.

They found significant, momentary lapses in several areas of the brain, which seemed to frequently falter when the people were deprived of sleep, but not when these same people were well rested.

'These people are not lying in bed. They are sitting up doing a task they learned and they are working very hard at doing their best,' Dr Dinges said.

He said the lapses seem to suggest that loss of sleep renders the brain incapable of fully fending off the involuntary drive to sleep.

He said the study makes it clear how dangerous sleep deprivation can be while driving on the highway, when even a four-second lapse could lead to a major accident.

'These are not just academic interests,' he said.