09/02/2026
This is your reminder, as we’re getting into cold and flu season, that it’s not really about the season. It’s about what the season does to your habits.
Less time outside means less sunlight, which means your vitamin D levels drop right when your immune system needs them most. Your immune cells actually need vitamin D to activate and go after invaders in the first place, it’s not just about your bones. And it’s not just the sun, it’s movement too. Colder weather means most of us stop walking, stop training outside, stop moving as much in general, and regular movement is one of the things that actually helps your immune system function well.
Then daylight saving time hits, and it’s suddenly dark at five in the afternoon. That abrupt shift throws off your circadian rhythm more than people realize. Hospitals actually report a spike in depressive symptoms specifically after the fall time change, not the spring one, which tells you how much that lost evening light matters.
Then there’s the stress. Elevated cortisol directly suppresses your immune system, so whatever’s going around, you’re more likely to catch it, and catch it harder.
And look at what actually happens this time of year. Halloween candy. Overeating on Thanksgiving. A month of Christmas sweets and holiday stress. Then New Year’s, staying out late, drinking more than usual. Every one of those puts more strain on the same immune system you’re counting on to fight off whatever’s going around your office or your kid’s classroom.
You’re not just unlucky this time of year. You’re stacking four months of behaviors that make you an easier target, and most of that is actually within your control.
So here’s what to actually do. Get outside every morning, even when it’s cold out. Keep moving, even if it’s indoors. Protect your sleep, especially the first week or two after the time change. Watch the sugar and the alcohol through the holidays. And manage your stress instead of just pushing through it.
