Today is "Blue Monday," which was invented back in 2005 as part of a marketing campaign for a travel company, by psychologist Cliff Arnall. At the time, he declared the third Monday in January to be the most depressing day of the entire year. Why? Well there's the freezing cold temperatures, your failed New Year's resolutions, your post-holiday debt, and basically everything is just terrible. He's explained in the past that the "low" is also caused by a drop in your motivation and "the hibernation effect"—in other words, we're eating and sleeping more, and basically just feeling lethargic.
Other experts have called Arnall's "Blue Monday" theory "stupid," and he has even since called it "meaningless." But we can say that 3 out of 5 Gothamist staffers have a touch of the blues today, so we're believers! Though... one is unsure if it's residual food poisoning from the weekend, and another may just miss her new cat who is currently all alone in her apartment :(