For Vitamin D synthesis, the sun needs to be at an angle of 50 degrees or higher. Depending on the latitude where you live, the sun may not get that high all winter so you may need to supplement.
No, the composition of sunlight literally changes throughout the day. When the sun is low in the sky in the morning or evening, light is traveling through more atmosphere than it is at noon. As light travels through the atmosphere, a certain fraction is scattered in a wavelength-dependent way. UV-B light--the Vitamin-D-causing kind--has a hard time passing through the atmosphere because of interactions with ozone molecules, so at grazing angles almost none gets through. However UV-A light is less affected by ozone and can still give you a sunburn. Interestingly, ordinary glass blocks both.
From memory, it's not just about sunlight, but rather specific band of uv-light; and the atmosphere filters it when the sun is at specific angles. It's not as simple as "just sunlight".
tim333|4 years ago
lodi|4 years ago
beagle3|4 years ago