I am not sure I entirely accept the concept of "opportunity cost". It certainly has problems, it depends on how free you really are, as has been nicely shown in one of my favorite videos: https://www.youtube.com/watch?v=VVp8UGjECt4

In any case, I think this was mostly resolved in modern society, with concept of overtime pay. (I assume, when talking about 6-hour work day, we are implicitly talking about employment.) No one is prevented working more, given that someone is willing to pay more (to the same person). And reason for this arrangement is precisely to give people the choice to cash on the free time or the extra value.