I also suffer brief crashes when beginning D3 supplementation or tanning. This is normal for anyone with low D levels. There are studies showing that initial supplementation actually suppresses the immune system for 24 hours before the benefits kick in. This may partly explain the crashes people with ME experience. I can't find the studies which show this - will continue to look for them. There was a graph with time line showing this effect. If someone comes across it please let me know or post it to this thread. Following on from this, if ME sufferers have very low D levels then the initial impact is much worse. However this should not deter you from continuing. In my experience the benefits of starting off very low, and gradually increasing dosage or time in the sun will provide benefits. Even then you will still go thru this up down scenario with gradually diminishing lows. Another part of the mystery is that both calcium and magnesium is important in the process. Guess what many with ME lack - magnesium. Additional supplementation is required. If calcium or magnesium levels are low in the body, your immune system will be suppressed. I will endeavour to find the studies of this as well. The point I am trying to make is that if you suffer a crash with D supplementation (and it should be D3 or sunlight in small doses) please persist.