Thanks African... Was starting to think I was some work-crazed monster 
My responsibility is to ensure that the job that needs to be done, gets done. "End of."
Glitter, I don't need to read that because we don't have any part-time employees, so it doesn't really apply to me. In everyone's contract, it says, quite clearly, that your contracted hours are 8:30-5:30, however, when the job requires, you may be asked to work extended hours, and in these cases, overtime will not be paid. It's quite clear what's expected of you from the beginning, so it's not as if people don't know what they're signing up for. If an employee feels they are being asked to work unfair hours, then that's an issue for HR, not for me. We're not always working 10-hour days, and we quite often have downtime in-between projects, but when it's needed, it's really, really needed, and we expect everyone to contribute to get stuff done. We don't ask people to put in 10-hour days for the hell of it. Of course there are exceptions, if you need to leave early for a doctors appt, etc., and I would never verbally ask someone to make up their time, but it is almost always done, either by working at home later that night, or by coming in early/staying later the next day. If it isn't, that is recognised and grumbled about by all of the other team members.
If one of my employees asked to cut their already busy 5-day work week to 4-days, it would logically follow that either someone is going to have to make up that work that isn't going to be done by her cutting a day, so that would pass the workload onto others, who are equally as busy, or that she would be working extra hours in those four days to make up for that fifth day. I don't know what the OP's work is like, so don't know if that applies in her situation, but I simply stated what the reaction would be should that happen in my workplace.
I don't want to work 10-hour days, but I do want the salary that goes along with it, so that's a trade-off that I, and all of my other co-workers and team members, choose to make.