Misery is expected to peak on Monday, as 24 January has been pinpointed as the worst day of the year.
January has been long regarded as the darkest of months, but a formula from a part-time tutor at Cardiff University shows it gets even worse this Monday.
Foul weather, debt, fading Christmas memories, failed resolutions and a lack of motivation conspire to depress, Cliff Arnalls found.
"Yes, we do see lots of people with depression and anxiety in the winter months.
"The message is it's not a terrible disorder, people do get better," Royal College of General Practictioners spokesman Dr Alan Cohen said.
"Exercise and bibliotherapy - reading a number of books to allow people to understand their own symptoms and how to control them," were initial treatments, he said.
The formula for the day of misery reads 1/8W+(D-d) 3/8xTQ MxNA.
Where W is weather, D is debt - minus the money (d) due on January's pay day - and T is the time since Christmas.
Q is the period since the failure to quit a bad habit, M stands for general motivational levels and NA is the need to take action and do something about it.
JANUARY BLUES DAY FORMULA
1/8W+(D-d) 3/8xTQ MxNA. Where:
d: Money due in January pay
T: Time since Christmas
Q: Time since failed quit attempt
M: General motivational levels
NA: The need to take action
Dr Arnalls calculated the effects of cold, wet and dark January weather after the cosiness of Christmas coupled with extra spending in the sales.
He found 24 January was especially dangerous, coming a whole month after Christmas festivities.
Any energy from the holiday had worn off by the third week of January, he said.
By Monday, most people will have fallen off the wagon or abandoned the nicotine patches as they fail to keep New Year's resolutions.
That compounds a sense of failure and knocks confidence needed to get through January.
The fact that the most depressing day fell on a Monday was not planned but a coincidence, he said.