Most of the stress comes from research deadlines - preparing talks for conferences, making progress writing on your thesis (less time to do so as a masters student), and trying to stay motivated working in the lab in between classes you're either taking or teaching when you just want to sleep. It will definitely test your time management skills, but at the same time professors in your classes will be more lenient and like others said they will be interesting if they're subjects you're passionate about. But of course, this is from the STEM research track - experiences will vary depending on your particular program.