WASHINGTON - A FEW nights without sleep can not only make people tired and emotional, but may actually put the brain into a primitive 'fight or flight' state, researchers said on Wednesday.

Brain images of otherwise healthy men and women showed two full days without sleep seemed to rewire their brains, re-directing activity from the calming and rational prefrontal cortex to the 'fear centre' - the amygdala.

'It's almost as though, without sleep, the brain had reverted back to more primitive patterns of activity, in that it was unable to put emotional experiences into context and produce controlled, appropriate responses,' said Dr Matthew Walker of the University of California Berkeley, who led the study.

That a lack of sleep can make people grumpy is hardly news.

'We all know implicitly the link between bad sleep the night before and bad mood the next day. We are just adding the brain basis to what we knew,' Dr Walker said in a telephone interview.

Dr Walker and colleagues at Harvard Medical School used functional magnetic resonance imaging, which can scan brain activity in real time, to see what was going on in the brains of their 26 young adult volunteers.

Half were kept awake for a day, a night and another full day. The other half slept as normal.

Writing in the journal Current Biology, Dr Walker's team said they noticed profound changes in the brain activity of those volunteers who stayed up.

'We found a strong overreaction from the emotional centres of the brain,' Dr Walker said. 'It was almost as if the brain had been rewired, and connected to the fright, flight or fight area in the brain stem.'

Swinging like a pendulum
And lab workers noticed a difference in the behaviour of the sleep-deprived volunteers.

'They seemed to swing like a pendulum between the broad spectrum of emotions,' Dr Walker said. 'They would go from being remarkably upset at one time to where they found the same thing funny. They were almost giddy - punch drunk.'

Next Dr Walker wants to test people who are chronically sleep-deprived, perhaps by letting them have just 5 hours of sleep over several days. The average adult needs 7 to 9 hours of sleep a night.

He said the findings may shed light on psychiatric diseases. 'This is the first set of experiments that demonstrate that even healthy people's brains mimic certain pathological psychiatric patterns when deprived of sleep.'

'Before, it was difficult to separate out the effect of sleep versus the disease itself. Now we're closer to being able to look into whether the person has a psychiatric disease or a sleep disorder.'

A second study in the same journal suggests daylight-savings time regimes may cause similar effects.