I thought this myself. December has the shortest days of the year in the Northern hemisphere, where I assume the poster is.
The effect is so strong that my hens, whose laying had been decreasing for a couple of months, would start increasing just before Christmas, within 2 or 3 days of the solstice.
I would definitely suggest mentioning it to a doctor. If the cause is physical, spiritual or psychological aids won’t help.
ETA: sorry about the mangled quote :o