I can provide my anecdote. I live near Portland, OR and been taking 5000IU of Vitamin D for years. It doesn't do anything to help with seasonal sadness. Neither do those "sun lamps". The only thing I've found helpful (although short-lived) for that is literally leaving the state for a week to somewhere warmer; like Hawaii or San Diego. The sadness comes back shortly after returning though, and sometimes even worse than before. It seems to me that treating seasonal depression is more complicated than simply taking medication or sitting in front of a lamp.