During winter, when daylight hours are short and the chill keeps folks indoors, many of us feel a little less upbeat. But blue moods that linger can be a sign of seasonal affective disorder (SAD), a type of mood disorder that can drain your energy, dampen your spirits and disrupt your daily routine. Fortunately, you don’t have to suffer through until spring. Supplementing with vitamin D for seasonal affective disorder can boost your joy and renew your vim and vigor.