I'm the co-founder of a start up and as you can imagine - it's quite stressful all the time.
To combat this, I either break something, eat something or go home for a bit. I also play the drums so that really helps :).
Something that I've started recently as well is a blog - it helps you vent your frustrations and clears your head. It really helps me. The feedback you receive really helps.
I understand that some freedoms aren't given to people lower down the ladder but I tend to find that as long as you're doing your job and doing it well then talking to your superior about flexible times and things - would be a good idea.