Lately, I've been making a conscious effort to get more sun (safely, of course, with sunscreen.) Most people in the ME/CFS community are familiar with the various studies about the links between Vitamin D and sun exposure - studies showing that certain neuro-immune conditions are far more prevalent in higher latitudes, where sun exposure is less. I won't repeat that literature here.
I've noticed that when I spend at least an hour during a given day with a signifiant amount of skin exposed to natural sunlight, I feel better in the evening and the next day. I sleep better too. Much better. Before I fall asleep, the feeling is of an increase in physical strength. I suddenly feel stronger and more virile.
Since very early after my diagnosis of ME/CFS, I have taken supplements of Vitamin D3. I've had multiple doctors express to me how critical it is for ME/CFS patients to have Vitamin D3 levels in the normal to upper-normal range. Under doctor's orders, I have supplemented with anywhere from 5,000 IUs to, at times, as much as 10,000 IUs of vitamin D3 per day. This has ensured that my vitamin D3 levels on blood tests have maintained in the range of what my doctors say is "optimal." It's difficult to say if these optimal levels contributed to the steady improvement I experienced from 2011 through 2015.
But I do know that supplemental Vitamin D definitely does not feel the same as the benefits I get from natural sunlight. I don't know if sunlight benefits me in some way besides Vitamin D (I can't imagine what that could be) or if the natural production of Vitamin D somehow trumps the supplemented version.
When we supplement Vitamin D orally, it must absorb from the stomach through the liver. By contrast, when we make Vitamin D through sun exposure, some of the vitamin forms on the surface of the skin and then absorbs through the skin. According to one of my doctors, it can take 24-48 hours for this Vitamin D to absorb, thus his recommendation that I not shower until at least 24 hours after a good sun exposure. (This is very difficult for me to do given the combination of sweat and sunscreen on my skin, but I have been trying to do it.)
The bottom line is, it is unmistakable that I feel better when I've gotten exposure to natural sunlight. Like everything I've found that helps, it's not a cure-all, but it's one small thing I can do to better manage this illness.