A variant of this question was asked in "Sacrifice Your Health For Your Startup" at http://news.ycombinator.com/item?id=679045 referring to http://blog.asmartbear.com/blog/sacrifice-your-health-for-yo...

pg had a good answer at http://news.ycombinator.com/item?id=679278

   I disagree with this. It's a net win to exercise. You're repaid for the couple 
   hours a week you lose with greater productivity during the remaining hours.
   This is the crucial mistake: 
   "How much time does a bootstrapped company take? All of it."
   That's not true. What a startup (bootstrapped or not) takes is 100% of 
   your performance, not 100% of your time. And optimizing for performance 
   means spending some time on maintenance.

I look at hobbies as maintenance for your mental and emotional health.