I had this conversation with my wife just the other day. If you work doing something you love, surely there must be some downside to it. What I came up with was time. Irrespective of what you do for work, what you do to relax, how long you sleep to recharge, or your interactions with family and friends. The only commonality is time. You have a limited number of hours in a day and how those hours are split is what keeps me at least sane. Spend too much on one and you find that one of the others will suffer.
My work-life balance is a constant juggle of that finite time. If I lock myself away continually writing, sure I feel invigorated, but I miss the interaction with my family. If I cut back on sleep, I don't concentrate well or I get cranky. Even if I was to spend time relaxing, to be honest, I would need a break because it would get boring after too long.
Thats my thoughts on a successful work-life balance. Know your constants, how long you need to sleep to feel refreshed. How many hours you need to work. How many hours you need to relax or how many hours you need to interact.
Managing that you can at least know where you are taking time from, when you are forced to do more in one area or another.