Consider it as a job, even when it's a hobby you're doing on your free time. This mean that you've the right to take a day off, the right to be sick, and the right to have some holidays. It can be difficult since you generally do it when you've finally the time for this, but taking at least a couple days off after each update should be something mandatory. You did a good job, you then deserve a reward.
Talking about rewards, it's also a way to go. When I was still freelancer, and she was still alive, my wife used to make my favorite meal each time I finished a contract. It's not something big, but morally speaking it always did marvels. So, you can also just treat yourself time to time. You worked your ass out all the week, really advanced on the update, time for a movie, but not on your TV screen, a real one in a theater, the good old "movie time". It will be 3 hours during which you'll not works on your game, but also 3 hours during which you get back the energy spend during the week.
What's important to understand is that, as long as it's not too frequent, each hour that you'll not spend on your game will increase your productivity once the break will end. You haven't lost 3 hours in my movie example, you've gained the energy to do an effective week of works ; opposed to a week of works when half drained, where you'll advance slowly and do less than expected.