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 center" - 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 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," Walker said in a telephone interview.

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. 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 centers of the brain," 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."