I guess it's because they might be shifting to a new schedule and it wasn't feasible to change sleep patterns? So if you get in at 00:00 one day, sleep until 08

I guess it's because they might be shifting to a new schedule and it wasn't feasible to change sleep patterns? So if you get in at 00:00 one day, sleep until 08:00. Then you don't get a new assignment until 02:00 the next day day, you will have the necessary gap, but not have had any real way to have slept the right time. You would have needed to force yourself to sleep in the afternoon/evening, and, even with drugs, that is a hard thing to pull off.