Having been based in various co working spaces where many startups work from I can definitely say most work regular work weeks!
As the other reply says, usually you find people doing longer hours in a few selected periods of a startups life. E.g. First MVP, first big client... crunch time!
Again, n=1 but I've experienced and have friends who have experienced around 50 hours/week for mid/later stage startups.
The stress is lower because you usually have more cash flow or significant rounds that make the work more enjoyable and you have a better comfort of working overall.
As the other reply says, usually you find people doing longer hours in a few selected periods of a startups life. E.g. First MVP, first big client... crunch time!