Here is a (rather long) comment I recently made on Terri Windling's blog, in reply to her wonderful and insightful and beautiful post on creativity, rest, burn-out and despair. Read the original post.
When I was studying to become a medical herbalist, I took some extra papers to gain a small certificate in sports nutrition, and in doing so I learned an interesting concept which comes to mind when I think about or experience burn out: when training to make your body stronger, faster, leaner, or to reach a certain goal, the body requires specified times of rest.
At night as we are asleep, our bodies calls forth the minerals and nutrients from the deep to strengthen, heal and facilitate growth of all those tissues that sustain us and makes us powerful -- our muscles, ligaments, tendons and bones. Just as we need the night and the sleep for this nocturnal regeneration and rejuvenation to occur, we need days off as well.
If two or three days of hard physical work is not broken up by a day or so of not doing anything physical at all, the body doesn't get better. The muscles do not grow larger. The bones do not get thicker. You don't make your goals. When you do not give your body the time it needs to repair your tissues -- time to make them larger, more durable and stronger then ever -- plateaus and burn-out are much more likely to occur.
I try and remember this to temper the inspiration, motivation and drive that I feel for my creative and work projects. If I feel my energy and my spirit wane, I have to let go of whatever important thing I am doing, so that I can go down and go deep into some rest, and have faith that whatever needs to get done, will still get done.