Somebody told me a good analogy only today. Imagine your body has 2 batteries, a normal running battery and a reserve.
Both batteries get recharged overnight when you sleep. If there are special demands, such as vigorous exercise or you have to go short of sleep, the reserve battery cuts in. But people with vasculitis don't have the reserve battery, so when the normal running battery runs out, there's nothing to fall back on. I thought that was good!