Even if you don't want to. Especially if you don't want to.
Go for a walk, go for a run, fork over the compost, rake some leaves, shovel the walk. Stand outside for five minutes on your lunch break and text your best friend. Just keep your eyes open so the sunlight can hit the back of your retinas, because apparently that's where it's needed.
There'll be days this winter when the sky never really lightens, days of sleet and cold and dark. But there will also be clear, sunny days, when the snow makes for a brighter light than ever summer brings. When those days come, don't spend the whole damn time indoors. If it's sunny, go outside.
The other first rule? Start early. Start now.