Yes and no. About a year before I started this job, I founded a digital publishing company with a friend, so I was working this 40 hr/week job AND on my startup whenever I "got done" for the day.
It got to the point where I finally took a step back and realized that I was killing myself. My crappy work schedule mixed with late, late nights on my startup was a recipe for disaster. I also started having some RSI issues, which I'm still dealing with today (not being physically able to work another minute is a pretty strong motivator to stop for the day... ).
Just let go - I know it's hard to do if you're working for a startup, particularly if it's your startup. But you need to draw the line somewhere - work a set number of hours a day, or work until a set time and then clock out. Work hard/smart during those hours, and you'll get the satisfaction of knowing you did what you could that day and it's enough.
Co-workers/founders are a terrific source of motivation - especially if they're constantly committing quality code. But don't let that drive turn into something bad, or the guilt will get to you and destroy whatever motivation you had for the project. Programming because you feel you have to is no fun and is not sustainable - you will burn out.