I found this presentation on sleep disorder by Dr. Stasha Gominak, MD Neurologist, very interesting and informative. She mentions PD couple time, but mainly explains how vitamin D3 (not D2) is crucial for our sleep, explain levels we need to have in our body for optimum results and test we should have if we take this vitamin as to little or to much is equally bad. There is an interesting explanation why rats use D2 which doesn't work for humans as we need D3. I wonder if that is why some medical experiments that worked on rats didn't do so well on humans. The presentation is over an hour long but well worth watching just because PD patients have usually low D3 level. https://www.youtube.com/watch?v=h7cbBB1c0IM