If you live above the 40th parallel — think Chicago, New York, London, Stockholm — there are roughly five months each year when meaningful UVB from the sun simply doesn't reach you. The angle of the Earth is too steep. The atmosphere filters it out before it gets to your skin.
This isn't a nuance. It's a hard biological fact. And for most of us, it means that from October to March, our light environment looks nothing like what our biology was built for.
It's not just about vitamin D
The common conversation about light and winter centers on vitamin D, which is a reasonable starting point. But the biological picture is much wider than that single nutrient.
Light — particularly morning light intensity and the presence of specific wavelengths — directly coordinates your circadian rhythm, your body temperature cycle, your cortisol awakening response, and the timing of melatonin production at night. When the input of bright morning light drops from a natural 10,000+ lux to the 200-500 lux of typical indoor environments, the clock gets imprecise. The rhythm loses its edge.
Most people experience this as a diffuse, low-grade shift: slower to wake up, harder to fall asleep, a slight drop in afternoon energy, a general sense of flatness. Not a clinical condition. Just a body quietly running on an insufficient signal.
The winter protocol — practically
The goal for the darker months isn't to replicate summer. It's to give the body a consistent, unambiguous morning signal that anchors the rest of the day's biology. Here's what a practical winter routine looks like:
Start in October. Don't wait for January
The most common mistake is starting light work in the depths of winter after already spending two months on a depleted signal. The optimal moment to establish the routine is before the deficit accumulates — when the days are shortening but haven't yet bottomed out.
Start in October. The routine will be established by the time it matters most, and you'll enter the darkest months with a rhythm already anchored.
Spring doesn't start when the calendar says it does. For your biology, it starts the morning you give it back its signal.