Wayne Allen wrote a good post on Going the Extra Mile – Why Bother?  There ensued a good discussion of various points in the comments, including this one from Johanna Rothman (quoted here in its entirety).

Early in my career, I would "sign up" for lots of stupid overtime, crunches, pushes to make the date. I postponed vacations, dates, and all kinds of other social events to "give my all" to the project. One day, I realized that working all that overtime just made me stupid and that I couldn't save the project.

Now, when a manager thinks that the answer to the project being in trouble is to add more hours, I ask him/her to think instead about what they can remove from the project to meet the deadline. The project can't complete as originally promised, so stop trying to make it that way. Instead, work like the devil (during the normal work week) to complete what you can. Sometimes my clients even do what I suggest :-)

Excellent advice.  One of the most important things to remember is that you are not defined by your job.  You could also be a spouse, a parent, a child, a friend, a volunteer for a charity, an awesome video game player, etc.