After a night of tossing and turning, we're definitely upset. And we admit to sometimes, maybe, taking it out on other people. Now, a new study explains why being sleep-deprived makes it so hard to just let things go. For the study, published recently in the Journal of Neuroscience, researchers recruited 18 people to have their sleep monitored for two nights. Participants weren't allowed to have alcohol or caffeine for two days before the study, and the researchers monitored their sleep patterns with a Fitbit for three days prior. Then, participants either got a normal night's sleep or were kept awake all night. At 8 a.m. the next morning, they had to go through two rounds of tests. For the first one, participants had to follow moving dots on a screen while the researchers monitored their brain waves. Pictures with positive (e.g. kittens), neutral (e.g. cows), or negative (e.g. snakes) associations appeared behind the dots to distract the participants. And for the the second test, participants performed a memory task while the researchers scanned their brains with functional magnetic resonance imaging (fMRI). Participants had to remember a series of numbers while also looking at those distracting positive, negative, or neutral pictures. Although the rested participants did better on the tests overall, results showed that all participants were distracted by the positive and negative pictures — a reaction the researchers categorised as an emotional response — regardless of whether they got a good night's sleep. But, if they were sleep-deprived, participants also reacted emotionally to the "neutral" pictures. Researchers confirmed via brain scans that these were indeed emotional reactions. So, sleep deprivation seems to remove your ability to tell the difference between the things that are worth getting worked up about and those that don't really require that reaction. And, as the study authors suggest, "[sleep deprivation] shifts the threshold for emotional activity to a lower set point, leading to the familiar 'all too emotional' feeling of sleepless nights." Unsurprisingly, other studies show that couples tend to fight more after sleepless nights, and previous research has found that, once we're sleep deprived, we may have some other unexpected mood and personality changes. For instance, going without a good night's sleep can make us feel less grateful and less optimistic. Also, weirdly, it can make it harder for us to find things funny. So preserve your friendships — and your sense of humour — by getting your necessary ZZZs.