It's fine in as much as it's your gig, it's on you to decide what is right or not. If you believe that working extreme hours at the cost of your own wellbeing and rational performance will end up being a benefit to you in the long run then it's "fine" for you to make that choice and you either reap the benefits of suffer the consequences and hopefully learn from it.

It's not fine to project your notion of drive and passion onto people who are employees and expect them to match your personal standards when the potential rewards are magnitudes different.

With that said, it doesn't mean they are not getting plenty of sleep. What it means is that when they are not sleeping, they are probably working. At the gym, they are probably thinking about work. At social events, they are probably thinking about work. Etc... It's just the nature of getting a company off the ground.