A recent study in the October 23, 2007 issue of the journal Current Biology discussed how sleep deprivation affected emotions. Here is an excerpt from livescience.com that discusses the research:

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.”