Parents of toddlers have known for years that tired kids have trouble controlling their emotions. But recent findings from neuroscientists at Harvard University and the University of California, Berkeley, extend far beyond temperamental tykes. After the researchers kept adult volunteers awake for about 35 hours, they found with MRI scans that sleep deprivation impairs the “rational” prefrontal cortex’s control over the amygdala, the brain’s emotion center. The result is the moodiness that often accompanies exhaustion, described by the team as an amplified response from the brain’s emotion hub. The study also suggested that sleep deprivation interferes with the ability of the prefrontal cortex to make logical decisions.