If you know someone who’s usually even-tempered but sometimes gets angry over little things…or someone who is frequently angry, there’s an explanation you might not have thought of but that now has been scientifically proved—he/she may not be getting enough sleep. And this anger effect can happen in short order—after just two nights of missing a few hours of sleep from, say, working late or staying up to care for a sick loved one.

In a recent study, half of a group of adults (ages 18 to 79) slept normally while the other half reduced their sleep by a total of four to eight hours for two nights in a row. Then they all did a series of product-rating tasks while exposed to irritating noise. 

Result: The more tired study participants felt, the more likely they were to react angrily to the irritating noises and, rather than adapting to the annoyance, to grow angrier and more distressed as time went on. 

One problem with sleep-induced ­anger is that it doesn’t seem to discriminate between “big deal” and “little deal”—in other words, it can cause people to lash out at others in simple, everyday situations. That makes it unlike ongoing anger over, say, politics, which might not be healthy but at least doesn’t cause most people to snap at others over trivial matters. If you ­sometimes ­overreact angrily and wonder why, the simple answer might be that you are tired. If so, excuse yourself from a confrontation that could escalate. If possible, take a short nap. You can improve your mood, restore alertness and reduce mistakes by sleeping just 20 to 30 minutes.