Psychiatric problems can trigger sleep issues, and now research suggests the reverse is true—that is, a lack of shut-eye can cause psychological disturbances. Matthew Walker of the University of California, Berkeley, and his collaborators studied 26 volunteers, 14 of whom spent 35 hours without getting a wink. All the subjects then saw photographs that went from benign (wicker baskets) to increasingly disturbing (tarantulas and burn victims). Brain scans revealed that when the sleep-deprived participants viewed more gruesome images, their amygdala showed 60 percent more activity relative to the normal population's response. “The amygdala seems to be able to run amok,” Walker says of that forebrain structure that decodes emotion. The boosted activity leads to a pendulum of emotions, from upset and annoyed to giddy, in moments. Among its many other functions, sleep may serve to prepare “our emotional brains for the next day's social and emotional interactions,” says Walker, whose work appears in the October 23 Current Biology.