I go to the doctor on Wed. Along with the other tests she runs, I want her to test my vitamin levels, especially D. Since the bout of flu, and dealing with shingles, I have been constantly tired. Real tired. Sleeping way more than normal. Day before yesterday, I increased my D some. It helped. Yesterday, I increased it quite a bit. It did me a lot of good. While I don't want to overdo it on D, once before I was so low, I had to take prescription strength (50,000 iu) of D for several months. I may need to do that again.