There's a saying that one should always work to live, not live to work, and I wholeheartedly agree.
In our deathbed, we won't be saying, Gee, if only I had worked more, I would have had a much more fulfilling life."
Instead, a happy life is measured by memorable experiences and time spent with loved ones. Yes, promotions and other personal accomplishments make us feel good about ourselves, but will they figure prominently in the legacy we leave behind? Probably not.
That's why we shouldn't let our jobs overtax us. It's fine to work hard, but not to the point of losing a lot of sleep and having it interfere with our personal lives. Once work trickles into your marriage/relationship and prevents you from enjoying life outside of work --whether that means playing with your kids or hanging out with friends -- that's when you know you may have to tone it down or get another job.
It may be that you're single and love your job so much that you don't mind spending 50 or more hours in the office each week. If that's the case, I salute you! But keep in mind that your situation may change eventually -- say, you get into a relationship with someone -- which would no longer make the long hours sustainable.
A telltale sign that it's time to scale back the hours or get a new job is when you see the job adversely affecting your health. No job is worth sacrificing your well-being for, even if it pays really well. One has little use for money if he isn't even in good health.
Work hard -- but play hard as well!