Are You Burning Yourself Out?

I've experienced burnout several times in my life at Google and on my startup. I don't think I've fully probably burned out working at Nest Labs, but I've come really close sometimes. And actually, maybe it's because of that that sometimes I end up burning out because I'm giving it everything. So how do you stop yourself from going down your own rabbit hole? That's one of my main tools is when I start making work such a priority that I cancel other things that are good for my mental health. If I stop going to the gym because I just want to work one more hour on this thing can be done... then if it happens once or twice every week its