This doesn't acknowledge weekends, which if you have the same 8 hours sleep and 4 hours "not for yourself", add more than 50% to the time you supposedly have "for yourself". (4×5 + 12×2 vs 4×7.)
Which doesn't necessarily change the point, but I'm not sure what if anything the point is supposed to be, and it seems an important omission.
Which doesn't necessarily change the point, but I'm not sure what if anything the point is supposed to be, and it seems an important omission.