If you are scheduled for 40 hours/8 hours day, that is what you should do. If you are scheduled for 39, then that's what is expected. If you want to remain eligible for benefits, you have to do at least 39 hours per week.
I hate the little note our TSM often includes with the daily log, "continue working your full scheduled hours". Really? I find it insulting that I would have to be told to work my schedule when I know that is expected. But then, I wonder if that is really a problem - people quitting for the day when they feel like it.