Our bodies make vitamin D when skin is directly exposed to the sun. Skin exposed to sunshine indoors through a window will not produce vitamin D. Cloudy days cut down the amount of vitamin D the skin makes.
Taking it directly from sunlight is very important. I mean most of people take supplements. Me too. I take Vitamin D all the time, because I understand its importance. I also order other supplements my body needs from the Canadian pharmacy. But it's very important not to forget about natural sources.