I don't know much about sleeping schedules, but what I know for sure is that Melatonin needs total darkness.
Melatonin is a youthening hormone ( "the hormone of darkness" ), that plays great role in anticancer prevention as well as stress and immunity.
If your melatonin goes down ( most of us after 30 y old are starting being depleted and it's good to supplement ), than your seratonin ( stress hormone ) goes up.
And we don't want that to happen!
Melatonin is also related to dreaming and study shows that it increases REM sleep time.
Most of the people talk about sleep ( how long, when ).
Not many of us have noticed that healthy dreaming might be as important as the above one.
And I believe that dreams after working 20, 30 hours on a row differ on various levels from the dreams that happen between, let's say 11 pm and 7 am every night.
Although, the topic is very interested!