I recently discovered that reading a good book about teaching revitalised me when I was tired. So now, rather than shelving my reading because I have other work to do, I sometimes pick up a book so I can build the energy to do my other work. As Schwartz (2007) suggests, I am better off managing my energy, not managing my time.
Tony Schwartz (2007) Manage your energy not your time. Harvard Business Review, 85 (10), 63-73.