I used to get symptoms associated with Seasonal Affective Disorder every winter. Now for two winters straight the Vitamin D supplements have minimized those symptoms.
During my second winter in Portland, OR, I started to feel chronically tired and nearly unable to wake up each morning, no matter how much sleep I had had. Once I realized something was wrong, I started Vitamin D supplements and within a day or two, I felt like a brand new person. It was really dramatic. Since then, I take them every year during the colder months.