Startups are marathons, not sprints; It's about making the right decisions most (if not all) of the time when building, hiring, marketing, etc. Fatigue and exhaustion mean bad decisions; There's plenty of research to back this up:

http://likesbuildingthings.blogspot.com/2011/12/i-know-many-...

People who are willing to take the risk of founding tend to be workaholics by nature, but you don't need to be a workaholic by nature to be a founder. It's bad for you longterm and it sets a poisonous precedent for your company culture.