Physiologists refer to something called a "sleep deficit," which is typically considered to begin accumulating essentially right after one becomes fully awake after first rising in the morning (or whenever). Sleep deficit in this sense is a measure of cognitive fatigue, and depending on how one's typical schedule goes, the closer to their "natural bedtime" one gets, the poorer they perform in cognitive tasks. And moral and ethical decisions are most definitely cognitive in nature. There's lots of research behind this, and it all supports the basic findings of the study in question: clock time is essentially irrelevant to how someone performs during their typical wake/sleep cycle.
I think an additional item to study is how flexible people are with the timing of their cycle. My dad worked rotating shifts for years and years, and seemed to accommodate to the new shift quickly, as did my wife when she worked nights. I, on the other hand, have had a huge problem with trying to change work hours more than just a bit; it took me over a month to get used to working from 10am to 7pm after spending many years on a 7am to 4pm schedule, and I never felt particularly sharp after about 5pm.