What causes bad moods? Why do we sometimes slip into angry fits and melancholy torpors? In general, happy moods have easy explanations – we know why we’re elated. But a bad mood often seems to arrive out of the blue, a gloomy weather pattern that settles in from everywhere all at once. All of a sudden, we find ourselves pissed off without a good reason, which only makes us more pissed off.
The standard theory of bad moods is rooted in a psychological quirk known as ego depletion. Pioneered by Roy Baumeister and Mark Muraven in the 1990s, the basic idea behind ego depletion is that self-control and willpower are limited cognitive resources. As a result, when we overexert ourselves in one domain – say, when we’re on a strict diet, or focused on a difficult task for hours at work – we have fewer resources left over to exert self-control in other domains. This helps explain why, after a long day at the office, we’re more likely to indulge in a pint of ice cream, or eat one too many slices of pizza. A tired brain, preoccupied with its problems, is going to struggle to resist what it wants, even when what it wants isn’t what we need.