I originally wrote this on Qt3 but I think this might be helpful to others who are wondering about the effects of working long hours.
Here are my views on the matter:
I think with most people, 55 to 60 hours a week is about the maximum before they start to eat up stored health/sanity points. And even there, over time, you will start to suffer burn-out -- the rate depends on how much you enjoy what you’re working on. But sooner or later, it’ll catch up on you.
Up until last year, my typical work week was 60 hours. That's about 10 hours a day, 6 days a week and I'd take Sunday off for family/video games/etc. I would regularly push a beyond those hours because I was ignorant of the gradual effects of working beyond a healthy dose had on me.
I got my first real taste of working really long hours In 2009, when Demigod came out. It is a multiplayer-centric game with servers designed to help connect users. Like many Internet-centric games, its initial launch overwhelmed our infrastructure and we raced to fix/add to it. That event started a bad habit on my part of doing all-nighters. As a result, my normal schedule rose from 60 to 80 hours even after the 100 hour weeks of the Demigod effort subsided.
Less than a year later, still really not recovered from the work on Demigod and with Impulse really starting to take off, I returned to working on a PC game called War of Magic. It was at the tail end of development and our retail launch window had been set to August. My hours returned to being over 90 a week during the Summer – one all nighter a week.
Here is the part I want to emphasize though: I believed my disposition and work quality seemed just as good as it normally was. I was wrong. From later talking to my coworkers, friends and family, later I realized that was not the case. According to them, I was like “a heroin addict, I looked strung out and was very difficult to be around.” Since I regularly engage with the public, this would have significant consequences.
That August, things got even more pressing and I had to increase my work to over 100 hours a week (two-all nighters weekly). During this time, I considered my disposition and the quality of my work to be just as good as it was at 40 hours a week. It’s like drinking alcohol– but without the pleasure. 
The point being, those who say that they can work 60 or 80 hours a week without it affecting them are dead wrong. I was one of those guys – for years. Luckily (or unluckily) the consequences for this became too obvious to ignore and I changed my work habits.
These days, we avoid crunch times like the plague. We no longer take retail availability into consideration for our product cycles so that we have the flexibility of just pushing the release date back. That’s how big a deal we consider excessive hours to be. It costs us less to push a date back than it does to deal with the consequences of having people work long hours.