Startups need productivity not man hours. If you're happy you might make a 'happier' product, be more likely to succeed with business partners, more charming to angels and VC's, more friendly with your friends and even a better lover with your lover.

Each person only has so much to give.. you can squeeze a lot at the beginning and loose later or take a steady pace and reach the goal with a lot less stress.

At a startup you should work whenever you feel like it (which is hopefully seven days a week starting at 11pm and going til dawn, with copious amounts of caffeine and loud music). But if you can get a lot done in four days and it'll keep you happy, work four days, you don't have any corporate culture to prove anything to.

Startup: "...pre-revenue cash burning startup."

Business/company: "...solid profits, plenty of users..."

By this definition, most YC companies would be startups while 37signals/Fog Creek/Google would be companies. Incidentally, Facebook is still a startup by this definition, despite its size.

So like you say, the startup has to do whatever possible, because as I say, they're on borrowed time. They onus is on them to demonstrate that their idea and execution is accepted by the market ("Make something people want"). In a business with customers, profits, etc, they've done that and it's just a matter or managing what has been created. When you get to that point (especially when hiring non-founders), you have room to be flexible.