Yes, circadian rythms can be adjusted over time, however, consider the typical freight pilot's sleep pattern. A GAM pilot would be awake at around 3am, heads in to work for a few hours flying and could possibly be ready for sleep again at 9am. A couple of hours sleep, then back to work, home in bed at about 10pm. This would change depending on the route flown.
Circadian rythms don't adjust very well to two varying sleep periods per day. This is different to someone who consistently works during the night and sleeps during the day. The body can adjust to that.