text: You can only make Vitamin D by sunlight exposure on the skin, so the more skin that's exposed, the more vitamin D you make.  Also, in the summer, the skin is higher in the sky - you can more sunlight concentration on the areas of the skin that is exposed.\n\n5 minutes of summer sun in shorts would give you ample Vitamin D. In winter, depending on where you live and on how bright and clear the sunshine is and the amount of skin you have exposed, you might need 20 or 30 minutes a day.