The three key ingredients to being the healthiest and fit you can be are eating healthy, exercise and sleep.
Think of it as a tripod. If any one of these are missing it's like removing one leg from the tripod. It doesn't hold up.
Yes, improving your life in any of these areas is certainly helpful and highly recommended. And you might have already seen amazing benefits from improving one area. So think how great you would feel (mentally and physically) if you were to do all three.