The "spoons" theory works perfectly well to describe coping with a disability, too. Metaphorically speaking, some days the silverware drawer's full (if you've been doing the dishes and being conscientious about it); other days you've got to hunt around in the sink to find even one you can rinse off and hope it'll be enough. I have Cerebral Palsy, and I found myself nodding while reading.
Not just lupus, fibro, or chronic illness...