In normal participants, the amygdala seemed to be talking to the medial prefrontal cortex, an outer layer of the brain that, Walker says, helps to contextualize experiences and emotions. But, in the sleep-deprived brain, the amygdala seemed to be "rewired," coupling instead with a brain stem area called the locus coeruleus, which secretes norepinephrine, a precursor of the hormone adrenaline that triggers fight-or-flight type reactions. "Medial prefrontal cortex is the policeman of the emotional brain," Walker says. "It makes us more rational. That top-down, inhibitory connection is severed in the condition of sleep deprivation. … The amygdala seems to be able to run amok." People in this state seem to experience a pendulum of emotions, going from upset and annoyed to giddy in moments, he says. "There seems to be a causal relationship between impaired sleep and some of the psychiatric symptomatology and disorders that we're seeing," says Robert Stickgold, an associate professor of psychiatry at Harvard Medical School who was not involved in this study. - www.scientificamerican.com