There's no question that people need their sleep: studies have linked a lack of shut-eye to everything from disruptions in the immune system to cognitive deficits to weight control.

In fact, psychologist Matthew Walker of the University of California, Berkeley, says that "almost all psychiatric disorders show some problems with sleep.'' But, he says that scientists previously believed the psychiatric problems triggered the sleep issues. New research from his lab, however, suggests the reverse is the case; that is, a lack of shut-eye is causing some psychological disturbances.

Walker's team and collaborators from Harvard Medical School reached their conclusions, published in Current Biology, after studying 26 healthy students aged 24 to 31 after either an all-nighter or a full night's sleep.

Fourteen subjects spent 35 straight hours without getting a wink before being rolled into a functional magnetic resonance imaging (fMRI) scanners where their brains were observed while they viewed a set of 100 photos that became increasingly disturbing as they progressed. Early slides were snapshots of an empty wicker basket on a table; the scenes changed as the series progressed, however, to more shocking settings, such as a tarantula on a person's shoulder and finally pictures of burn victims and other traumatic portraits.

The researchers mainly monitored the amygdala, a midbrain structure that decodes emotion, and observed that both sets of volunteers had a similar baseline of activity when shown the innocuous images. But, when the scenes became more gruesome, the amygdalae of the sleep-deprived participants kicked up, showing 60 percent more activity relative to the normal population's response. In addition, the researchers noticed that more than five times more neurons in the area were transmitting impulses in the sleep-deprived brains.

Walker described the heightened emotional response in the weary as "profound," noting, "We've never seen a magnitude of increase between two groups that big in any of our studies before."

The team also checked the fMRI readings to determine whether any other brain regions had a similar pattern of activity, which would indicate that the brain networks were communicating with one another. 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. He cites research linking sleep apnea, in which breathing is disrupted, to attention deficit hyperactivity disorder and the evidence of a connection between depression and insomnia as examples. "It might be that those medial frontal regions tell the rest of the brain, 'You can chill,'" he says. "Those circuits become exhausted or altered after a lack of sleep."

Walker says the team now plans to examine the effects of disruption of certain types of sleep, such as REM sleep or slow-wave sleep. "I think we may start to think about a new potential function for sleep," says Walker. "It does actually prepare our emotional brains for next-day social and emotional interactions."