When you're tired, it can seem as if everything is filtered through a negative lens. That might be because your positive lens is fuzzy when you are sleep-deprived, according to findings of two related studies published last year in the Journal of Psychosomatic Research. Two groups of college students completed a series of tests to assess their emotional responses to negative and positive pictures, similar to those often shown on the nightly news. Twenty-eight students completed the test at various points throughout a night of simulated shift work after getting five hours of sleep, and 31 students did the same after staying awake for more than 24 hours. The students completed a short survey to indicate how each picture made them feel.

The researchers found that all subjects' emotional response to the photographs became increasingly dampened as the night wore on, and their reaction to positive stimuli was even more subdued—that is, they grew less likely to feel good in response to uplifting pictures. “The human brain is naturally more attentive to negative events,” perhaps as a survival mechanism that keeps us on the alert for life-threatening situations, says study co-author June J. Pilcher, a psychologist at Clemson University. Yet in modern society, life-threatening events are fairly rare, so instead we find ourselves overreacting to the pile of dirty dishes—especially when we, ourselves, are washed-out.