Sleep deprivation can cause groups of brain cells to fall asleep in rats who appear to be awake, and rats with sleeping neurons make more mistakes, new research has found.

Giulio Tononi, a neuroscientist at the University of Wisconsin-Madison., and his colleagues reported their findings Wednesday in the journal Nature.

Tononi's team kept rats awake for four hours beyond their normal bedtime, and recorded the activity of brain cells called neurons as they became more and more sleep-deprived.

Outwardly the rats appeared to be completely awake, but the brain cell recordings told a different story.

It is well known that nerve cells, including those in the brain, can exist in two states: their awake functioning "on" state, and an "off" state in which they are unresponsive.

During normal non-dreaming sleep large groups of neurons oscillate a few times a second between the two states, giving rise to the "slow" waves (characteristic of sleep) that can be seen on an electroencephalogram (EEG).

The longer the rats stayed up, the more cells would start to flick into the off state for brief periods.

Interestingly, the researchers found neurons in one small area could be asleep, whilst in another part of the brain they were awake.

"What is distinct about this research is that they have looked at an animal that has been forced to stay awake," said Christopher Colwell, a neuroscientist at the University of California, Los Angeles, whose research focuses on circadian rhythms.

"This is something we all have to do sometimes, and research has consistently shown that performance goes down. This can be very important for some jobs — air traffic controllers for instance."

Tononi's team also made the rats learn a new task while they were sleep deprived, getting them to reach out for a sugar pellet.

They noticed that the rats were more likely to miss the pellet if some neurons in their motor cortex, which controls movement, had been in the off state shortly before they reached out.

"This is exciting because it raises the possibility that what is happening when performance goes down is that some of the cells involved are moving into the sleep mode," says Colwell, "and we have every reason to suspect that what is going on in the rats is happening in humans too."