So I am pretty sure I have that Seasonal Disorder, but this year in spite of the mild climate I felt crappy. All my girlfriends were finding out weird things with their thyroids, so I thought I'd get blood work done.
Normal Vitamin D levels are between 30-100. When the results came back the doctor informed me that I was at 17- SEVENTEEN. Like, 'you're so far away, you're not even a dot!' kind of results. So he told me to take 2000 I.U. for 3 months and get back to him. After 1 week I felt so much better (aside from calcium absorption issues, low Vitamin D causes muscle fatigue); I felt like I was on speed.