Have you ever worked the night shift?  Getting used to the schedule isn't much fun.  It means eating supper and going to bed when everybody else is eating breakfast and getting ready for the day.  Days and nights are reversed.  It means sleeping when others are awake, working when others are playing, watching the moon instead of the sun, working in the dark instead of the day.  It's different.  Not everybody can take it.  Too lonely, too dark, too much of a difficult thing.