I really, really, really want people to avoid what I went through, so it thought I'd share some gory details. I clearly went through every phase of burnout jacquesm mentions.
A compulsion to prove oneself: This was my entire purpose in life. I worked for a consulting firm with an 'up or out' culture, and this really fed into my desire to prove myself. It's like getting addicted to leveling your character in an RPG, except in real life.
Working harder: My only mechanism for coping with stress was to 'work harder', which of course fed right back into this vicious cycle. There was a point when I saw how much work was ahead of me on my project, and it was distressing. I didn't think I could keep up the pace. I spoke to someone close to me and said as much, and they said sometimes this is what is required in a job; just break it up into pieces and bulldoze through it. That's the advice that stuck with me for the next year. That advice fed into all of my personality flaws.
Neglecting one's own needs: At one point I told my boss I needed a weekend off. He was a bit shocked since I hadn't had a weekend off in months. I remember still taking calls on my 'weekend off'. I worked from home, but not so that I could take a break, but instead so I could waste less time commuting or dealing with stuff like personal hygiene, (I'm not joking, I actually rationalized this). My schedule was wakeup, grab breakfast and laptop and start working. Work until I was hungry, then stop to make myself a sandwich and have a 'working' lunch. Around dinner time my wife would feed me, and I would usually work straight through dinner, if I didn't I would spend most of dinner talking about work. Then I would typically work until I fell asleep. I did this for months.
Displacement of conflicts (the person does not realize the root cause of the distress) Near the end I fabricated paranoid stories about my coworkers and their attempts to make me fail. It took me a year of recovery to finally realize these stories were false and my failure and eventual collapse was almost entirely my own fault.
Revision of values (friends or hobbies are completely dismissed) It's hard to have friends when you are working 80 hours a week. I didn't have a social life and I killed all hobbies.
Denial of emerging problems (cynicism and aggression become apparent) -- See displacement of conflicts
Withdrawal, behavioural changes, substance abuse, depression I went from occasional smoker to smoking a pack a day. I gained 40 pounds. I became physically ill for weeks at a time, (a simple cough took me a full month to get over). And..(and this is humiliating, but I'm hoping other people will learn from my mistakes), I cried myself to sleep most nights. I don't cry, but during this period of my life, I did.
Nothing is this important. It takes years to recover. If you are still convinced that you need to 'optimize your productivity' then know that my output dropped to zero and stayed there for a long time. That is not very productive.