Be aware... in the US, unless you are a salaried employee the law requires overtime be paid. Employees which are salaried often receive higher salaries with the understanding that they "work until things are done" disregarding any actual hours. Hourly-paid employees must be paid for anything above and beyond their scheduled work hours. (or 40hrs a ...
Looks like you have 3 options:
Find another job that would better fit your schedule and needs.
Try to discuss this with your superiors and hopefully better define your daily schedule.
Accept this job for what it is, although it doesn't sound like a very healthy environment.
You probably don't want the third option, so..