I know a few people who work 3rd shift. That is people who good reason to be up all night in their local timezone. They all sleep during times when everyone else around them is awake. While this is a small minority, this is enough that your scheme will not work.
I actually was considering those people. That’s part of why I suggested it shouldn’t be a hard cut-off, but just adding to the end of the messages.
Of course, one could add some sort of daily schedule feature thing so that if one has a different sleep schedule, one can specify that, but that would be more work to implement.