Energy levels just right, a normal amount of restful sleep, and the ability to focus on one thing. When I'm depressed I can't focus at all and I'm just too tired to think. When I'm hypomanic I keep switching between different things. Only when I'm euthymic can I just concentrate on and enjoy one thing. Also, self-esteem and my general outlook on life are major factors. They're only balanced when I'm euthymic, otherwise I get too pessimistic or overconfident. There's probably more, but this is what comes to mind right now.