There are lots of things that can sap energy. Poor diet - I know I've felt like crap the last couple of days because I've been helping myself to too much of the Halloween candy around the library. Lack of light - even in the summer if you don't get out in the sunlight. Lack of exercise. Lack of quality sleep. Stress. Even just boredom.

Have you had a physical recently?

P.S. I've just noticed - you have exactly 1000 posts on this forum, Kureha!