If you felt especially down yesterday, there might just be a reason for that. According to a British psychologist who in 2005 devised an equation comprising variables such as weather, debt, the time passed since Christmas, failed New Year's resolutions and other factors, Blue Monday, the last Monday in January, is the most depressing day of the year.
Okay, so the methodology used in deriving this date is dubious, and the purpose behind it - it was promoted by a British travel agency to encourage people to take a January vacation - is clearly suspect.
That being said, I think there is truth to the idea that the first few weeks of the new year are especially dreary and gloomy (I personally feel that January along with August are the two most depressing months of the year). Coming off the excitement and festiveness of a holiday season that begins building up well before Thanksgiving, culminates at Christmas, coasts through a week to New Year's Eve and then fades away, it's only natural that people experience an emotional letdown as they put away their Christmas decorations and return to the tedium of daily life.
(In that regard, perhaps it's unfortunate that, aside from Louisiana where it marks the beginning of Mardi Gras season, Twelfth Night/Epiphany is not celebrated in American culture outside of some churches. This event could have psychological value as a means of formally saying "goodbye" to the holiday season, but that's the topic of a whole other post.)
Add to that the prospect that a few more months of winter remain ahead (Seasonal Affective Disorder is a real phenomenon), as well as the fact that, yes, those credit card bills from Christmas shopping also come due, and it's easy to see why January could be considered an emotionally depressing month.
I've always wondered if a secondary reason why Martin Luther King, Jr.'s birthday was made a national holiday was to address this lingering gloominess. The obvious purpose of this holiday, of course, is to honor a great American. But perhaps an unspoken, lesser motivation behind this observance was to brighten people's spirits by giving them a three-day weekend during an otherwise dull and dispirited time of the year.