"Uh, the cultural origin is about religion, which I haven't seen much evidence of around here, but it persists in secular civilization - mostly I just notice you have a much reduced need for rest, and if you wanted more leisure time, you could do a lot of it at night. In societies where everyone is inevitably going to need time off because they're sick, or taking care of sick family members, or catching up on their grocery shopping or their sleep or their exercise, there are plenty of professions that need to be available all week long and so days off aren't always on the weekend, so between that and the way it'd be easy for you to slosh time off around freely, I'm surprised the concept hasn't kind of evaporated."