When exactly does rickets set in?

Just curious. As part of the house re-do, we swapped the home office and the bedroom. So now I spend most of my day in what essentially is a unlit cave. I'm getting a lot of work done, and it's not like you can get vitamin D from sunshine through windows (if I remember the Glamour investigation in the matter, the glass filters it out). But still....

I am starting to think about how I can incorporate more exposure to actual daylight in my life. And I'm not exactly sure what that says about me. Probably that whatever ties I had to nature are long-gone. (Private note to The Texan: See? See why I blanche at the idea of camping in the woods???)

Today, for example, I walked to the gym so that I could get some sun on the way over and back. I practically passed out on the way home, as it's a really muggy, hot day and like the brainiac I am, I was wearing all black. But dammit, I'm holding off rickets for at least another week now.