Being ambitious and having an environment that lets you run (and rewards you for it) is what prevents burnout at a place like that.

A lot of people at hedge funds like having a baseline level of good stress in their lives. It's only when the stress starts to make you feel insecure that it creates problems.

I worked at one of these firms for a while, and my "burnout" point came when I realized I wasn't actually getting rewarded for my extra contributions to the company (my bonuses were going up by a small amount each quarter, no matter what).