Without sleep, the emotional centers of our brains dramatically overreact to bad experiences, research now reveals.

"When we're sleep deprived, it's really as if the brain is reverting to more primitive behavior, regressing in terms of the control humans normally have over their emotions," researcher Matthew Walker, a neuroscientist at the University of California, Berkeley, told LiveScience.

Anyone who has ever gone without a good night's sleep is aware that doing so can make a person emotionally irrational. While past studies have revealed that sleep loss can impair the immune system and brain processes such as learning and memory, there has been surprisingly little research into why sleep deprivation affects emotions, Walker said.

Walker and his colleagues had 26 healthy volunteers either get normal sleep or get sleep deprived, making them stay awake for roughly 35 hours. On the following day, the researchers scanned brain activity in volunteers using functional magnetic resonance imaging (fMRI) while they viewed 100 images. These started off as emotionally neutral, such as photos of spoons or baskets, but they became increasingly negative in tone over time—for instance, pictures of attacking sharks or vipers.

"While we predicted that the emotional centers of the brain would overreact after sleep deprivation, we didn't predict they'd overreact as much as they did," Walker said. "They became more than 60 percent more reactive to negative emotional stimuli. That's a whopping increase—the emotional parts of the brain just seem to run amok."

The researchers pinpointed this hyperactive response to a shutdown of the prefrontal lobe, a brain region that normally keeps emotions under control. This structure is relatively new in human evolution, "and so it may not yet have adapted ways to cope with certain biological extremes," Walker speculated. "Human beings are one of the few species that really deprive themselves of sleep. It's a real oddity in nature."

In modern life, people often deprive themselves of sleep "almost on a daily basis," Walker said. "Alarm bells should be ringing about that behavior—no pun intended."

Future research can focus on which components of sleep help restore emotional stability—"whether it's dreaming REM sleep or slow-wave, non-dreaming forms of sleep," Walker said.

Many psychiatric disorders, "particularly ones involving emotions, seem to be linked with abnormal sleep," he added. "Traditionally people mostly thought the psychiatric disorders were contributing to the sleep abnormalities, but of course it could be the other way around. If we can find out which parts of sleep are most key to emotional stability, we already have a good range of drugs that can push and pull at these kinds of sleep and maybe help treat certain kinds of psychiatric conditions."

The findings are detailed in the Oct. 23 issue of the journal Current Biology.