Before continuing, I'll acknowledge that moods might seem like an unsophisticated subject for serious research. They strike at random and often pass in minutes. People typically shrug them off, along the lines of "oh I'm just in a bad mood," or "my boss is in a weird mood."

But the reason moods matter is precisely that they are so present. Since humans are terrible at thinking about the future, they make lots of decisions on the basis of how they feel, here and now, rather than how they're likely to feel in the future. New Years puts a person in a forward-thinking mood, which results in hundreds on a gym membership. Birthdays encourage thinking about today, which licenses indulgence.

Moods, despite their short lifespans, shape a person's attention, his entertainment, and his choices.

Negative moods can lead to a procrastination doom loop, in which an individual perpetually delays important tasks while waiting for an angel of inspiration to visit. Negative moods can lead to other doom loops, too. Optimistic and happy people are associated with higher incomes, more successful social interactions, and longer lives. Does the physical experience of happiness confer magical health benefits? Perhaps. But a popular explanation for the happy and successful is that positive moods can make the person exquisitely sensitive to rewards in an environment. A good mood heightens the benefits of going to the gym, eating raw kale, and doing favors for friends. A foul mood makes the individual exquisitely aware of all the downsides of these activities—the sweat, and bitterness, and the hours hauling couches up flights of stairs.

But there are some moderate upsides to bad moods: They heighten attention to detail. “If attention is like a spotlight, then a good mood will widen that spotlight, while a negative mood will focus it very tightly,” said Adam Anderson, assistant professor of psychology at the University of Toronto. If attention is a filter, good feelings widen the mouth, opening up to all sorts of stimuli, which helps with free association and creativity. But "that kind of broad or diffuse attention can be detrimental in situations that demand a laser-like focus,” Anderson said. Positive moods inspire faster and more creative thoughts, but fast thinking isn't always suitable for every task. For this reason, if a day includes both brainstorm sessions and Excel work, science would advise playing happy music before the group session and transitioning to a moodier fare for the detail-heavy work.

Moods can also determine the sort of products that a person finds interesting. A recent study in the Journal of Consumer Psychology investigated how happiness affects consumer choice. There are two main flavors of happiness, the researchers said: a present-based happiness, which feels like calmness, and a future-based happiness, which feels like excitement. These flavors evolve as a person ages. One study of 70,000 instances of happiness on blogs showed the meaning of happiness evolving over time from excitement when people are young to peacefulness when they old.