Whatever you do, constantly be looking for ways to actively mitigate it. Just working less is NOT the only way to prevent it. Everyone has had times when they've worked long hours on something, and still felt great. Think about what was special in those situations. Ideas:
1) Play a sport.
2) Party hard, roughly 1 night per week, maybe Friday. This works well for many extremely hard working professionals in NYC. Not sure how well it would work in SV...
3) When coding, try to break up tasks into self-contained chunks that you can start and finish within one day. Accomplishing a new tangible goal each day can have a great mental effect. Catching up on big half-finished tasks from the previous day can be stressful and mentally draining.
4) Be around people you love being around. You'll feel less stressed about escaping the office if the people you're looking forward to being around are inside instead of outside the office.
5) Notice how it's so hard to focus late into the evening at the office, but once you're home you feel awake again and can work? For the earliest stage startup, consider making your work environment more like a home, or literally work out of your home with your cofounder. Your girlfriend/boyfriend may look down at you for doing this, get used to it. Partners hate startups.
6) Go for walks twice per day, preferably outdoors, in a place where you're surrounded by lots of people - or alone in nature, whatever you feel like you're missing.
7) Kick out whining people, immediately.