This is the second year that I've fallen off the blogging wagon in starting mid-February to late March, and I think you can look back to previous years and see at least a fall-off at the same time. The problem has gotten more pronounced since I've moved to mostly office work.
Back when I was out in the field all the time, I had access to light in the winter. Sure, it was weak winter light, and there were short days, but I was absorbing something even if I wore a million layers and had no skin exposed. After all these years, it's clear I have a bit of seasonal affective disorder (SAD) and everything sort of... grinds to a halt after a couple months of darkness. I keep writing for a couple months after November/December out of stubbornness, but then it's so hard to get motivated to keep up.
I don't have any real solutions, except to try and come back up for air sooner rather than later. But in the meantime, I'll keep plugging away.