Does labor law apply to graduate students?
Graduate students fall into a gray area with respect to American labor law, because they are a bit of a hybrid between students and employees. At least for the first year or two, they are usually mostly taking classes, and doing some research, but the balance shifts more toward research in later years. 

As for the number of hours worked:


  Research ideas don't wait until you punch in in the morning.


The expected workload completely depends on the culture of the lab. I knew of groups in my old department where there really were no "expected schedules," and other groups where the expectation was about 80 hours per week! 

That said, graduate students in my department worked a wide variety of different schedules. Some were night owls, who showed up around 10 or 11 at night, and stayed until 8 in the morning. Others were punctually showing up around 9 am and sticking around to 5 or 5:30 in the evening—and everything else imaginable.

Personally, there were days when I worked from 8:00 to 6:00 or so; other days around deadlines I might work until midnight; still other days I'd take a half-day or so.