They say that happiness leads to productivity, and I don't think it works the other way around. While being productive can make you feel less like a couch potato and it's good for you to be moving around, productivity for the sake of productivity doesn't guarantee happiness. In fact, sometimes people use productivity as an escape for their unhappiness because as long as you keep busy you don't have to think about the things that bother you.
Doing something that makes you happy makes you feel more connected to yourself. It makes you feel better about what you're doing and who you are and even the environment you live in. The better you feel, the more you'll end up doing. You're less likely to experience burn out and the quality of work you put out will be better.
So if you'rein a rut, try learning about something you've always wanted to know. Take some kind of exercise class, like yoga or dance. Get involved in an activity that brings you joy, like painting, knitting, or playing an instrument. Step outside of your comfort zone and don't be afraid to fail from time to time. Taking time for yourself to do these things is so much better for you in the long run.