Lots of folks don't work at the same time daily, and more work a shift that makes them sleep less due to children (Second shift, but has to get up to get the kid off to school). A good deal of other folks have long commutes or can't get out after 8 hours (their shift is 8 hours plus an hour lunch or standard 10 hours).

It doesn't seem that hard to get unhealthy sleep patterns in a workday.