Vitamin D is necessary for a whole range of processes in the body, the most important of which involve improving the absorption of calcium for healthy and strong bones; maintaining a healthy mood; and supporting a healthy immune system. Seasonal Affective Disorder, otherwise known as SAD, is a common disorder during winter time, caused by the shorter days and lack of sunlight. We need at least fifteen minutes of direct sunlight each day in order to keep our Vitamin D at healthy levels. Due to the fact we spend a lot of our time indoors and lead sedentary lifestyles, we need to supplement with Vitamin D, especially throughout winter time, since it is virtually impossible to get enough Vitamin D from the sun during this time. Get outside whenever possible. Even if you only expose your hands and face to the sun during wintertime, this can still be sufficient to produce some Vitamin D. Egg yolks, butter, and cod liver oil are good sources of Vitamin D.