Gift Peddie's comment goes a good way to explaining why I'm a contractor. I do my hours and walk out the door. No more working for free - that's for suckers.
Gift - People get exploited because they allow themselves to get exploited. It's insidious - bad managers first give you more work than you can complete in your regular work hours, then when you meet the objectives, they pile it on further. With such treatment, sooner or later they'll be a crisis when you can't deliver. So you're better off to precipitate the crisis earlier before it affects your health.
So you actually have four options:
1) Quit and get a job elsewhere. If you don't like the idea of being out of work and aren't confident in your ability to secure work, you'll never make it as a contractor.
2) Precipitate a crisis by failing to deliver within the unreasonable time constraints. If you're doing considerable overtime and you're technically competent, what are they going to do? Fire you? They won't be able to find anyone else who'll do so many hours for free and then there's the learning curve of picking your stuff up - it won't happen.
3) Suck it up, be unhappy, be stressed and fall ill.
4) Go and find a job in a jurisdiction that doesn't permit excessive overtime, like the Netherlands, i.e. get out of the USA! In the EU, there's the Working Time Directive which puts a limit on the number of hours any employee can work. Some countries (like the UK) have clauses where an employee can contract out of its provisions, but many others don't, and not all the EU countries have poor economies.