It doesn't really make much sense to talk about the length of a work day except on average. "Six-hour work day" should just be interpreted to mean "120 hour work month" or similar.
Many workers have jobs where human presence is a key part of the job function. Being a cashier, or a nurse, or driving -- it isn't as though these things happen unless you are physically there. The quantum of human presence is "one human" :-) so even if there are no customers, the driving is boring, or the patients are asleep, it still requires you to be there on the job.