Anyone who's ever had a poor night's sleep knows it can make them more cranky than usual the next day. Now researchers think they know why: there are strong neurological links between sleep and emotions.

To find out, researcher Matthew Walker from the University of California at Berkeley asked a group of people to stay up for 35 hours at a stretch. Then he put his subjects in a brain-scanning machine while showing them photographs. The first photos were 'neutral': pictures of a leaf or of a basket. But as the parade of images continued, the pictures become more disturbing: such as photographs of a shark attack, or severed limbs.

Walker says that affected deep emotional centers of the brain, specifically a region called the amygdale. "The amygdale were actually up to 60 percent more reactive to those emotionally-negative pictures in those who hadn't had a good night of sleep compared to those who had," Walker says. "And that really is a profound amplified emotional brain reaction to these negative pictures."

Walker says scientists know that the amygdale is involved in emotional processing. But he says this level of amygdale activity in the tired subjects is similar to activity recorded in the amygdalas of people who have some psychiatric disorders such as depression or post-traumatic stress disorder.

"It's actually very difficult to find a psychiatric disorder with emotional or mood symptoms that doesn't have impairments of sleep," asserts Walker. "And we've been struggling with the battle between the chicken and egg question. Is it the psychiatric condition that's causing the sleep impairment, or is it the sleep impairment that's causing the psychiatric condition?" Walker says it's also possible to induce psychosis in a healthy person who's been denied sleep.

He speculates that one function of sleep is to restore the emotional circuits of the brain, and says we need to pay more attention to the emotional and mental health benefits of sleep. "And I also don't think it's a coincidence that many anti-depressant medications, for example, actually seem to alter sleep and try and rebalance sleep," he adds.

Walker says the next challenge is to determine what kind of sleep and how much is most important for maintaining emotional health. His research is published in the journal Current Biology.