Sun is good, but high temperatures are bad. For me, cold but sunny winter days have a good effect and I have reduction of symptoms. Now that the summer has settled in, I feel worse.
My doctor, looking at a bunch of lab tests, told me, among other things, that my white blood cells are a little low - for that, it is better to be at cold temperatures, this increases the amount of white blood cells.
My understanding is that the immune system fights in some way (either autoimmune reaction or some inflammation that is taking place), so these white blood cells go "in vain", they are used unnecessarily to fight against something they shouldn't. But, it is in any case better to have more of them. The body produces more in cold temperatures. This is my view.