LPT: Sitting by a window doesn't help your body make Vitamin D. You have to get unfiltered sunlight on your face.

Every winter, I hear people mention that they're getting enough Vitamin D because they spend 15 minutes in front of a window each day, even in winter. It's good for your overall health to get any sun, but regular windows block most of the ultraviolet B radiation, which is the real ingredient required for your body to produce the hormone we call Vitamin D.

It's a good reason to go outside, even in the dead of winter. Not getting any unfiltered sunlight for 2 months at a time just isn't healthy, so all that time driving and working needs to get supplemented.

UV resistance of glass: Prevention: "Most of the commonly used glass types are highly resistant to UV radiation."

Vitamin D wiki