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.

You don’t know many full time workers who struggle with sleep?

Sometimes I find myself awake and alert after 4 hours of sleep. Now when I crash some hours later after that, I sleep for a couple more hours and work more after instead of suffering through meetings and commute without break. I no longer use an alarm and let myself wake naturally.