Your comment on feeling down keeping you from exercising made me think of a quote from Dan Millman, that in my experience, is certainly true. I read it that without releasing energy through exercise or some other positive way, it just starts to fuel negative emotions. so it's some of that "life energy" that needs to come out through exercise to help maintain a positive mindset. Life energy must flow somewhere. Where internal obstructions lie, the energy burns, and when it builds up beyond what an individual body/mind can tolerate, it explodes. Anger grows into rage, sorrow turns into despair, concern becomes obsession, and physical aches become agony. So energy also can be a curse. Like a river, it can bring life, but untamed, it can unleash a raging flood of destruction.