The Vikings won on Sunday, so why was Monday still the most depressing day of the year?

No, it wasn't because of the bone-chilling, subzero temperatures outside or the fact that you ate half your bodyweight in chicken wings and now, as a result, have moved a notch in the wrong direction on your belt.

The real reason you were depressed? Well, because a non-scientific, mathematical formula suggests you were.

Monday, Jan. 15, 2018, was officially "Blue Monday." Blue Monday is name given to a day in January, typically the third Monday of the month, claimed to be the most depressing day of the entire calendar year. The concept for Blue Monday was first published as part of a 2005 press release from holiday company Sky Travel, which claimed to have calculated the date using an equation.

The formula for deciding what day is Blue Monday uses a number of factors, including weather conditions, debt level, time since Christmas, time since failing your New Year's resolutions, low motivational levels and feeling a need to take action.

After punching this year's numbers into the equation, Monday, Jan. 15, was the result. In 2017, Blue Monday occurred on Monday, Jan. 16.

Anxiety disorders, including depression, are the most common mental illness in the U.S., according to the National Institute of Mental Health.

Anxiety disorders affect 40 million adults in the U.S. age 18 and older, or 18.1 percent of the population each year.