From my observation an ensign is about the same level as green third mate.
If the Navy operates the same way as the big cutters in the CG than what happens is all the officers are up during the day for operations, training and so forth. At night the senior officers all knock off and the ensigns stand watch at night. By contrast MM has the green third mate knock off at midnight and the more experienced second mate and C/M stand watch.
The Navy system is going to be stressed in the case of Japan coast wise if the ship crosses the coastwise traffic lanes after midnight.
As master I would not allow a green third mate to stand watch crossing the lanes, they lack the skills to do it safety.
My system can be “broken” in two ways, the first is if I am in an unfamiliar area and do not know when I should be on the bridge and the second is a back-up to failed planning, that the mate calls me when in heavy traffic or can not meet required CPAs.
The Navy system failed because of high tempo operations, planning should have anticipated crossing the lanes, increased tempo has also caused training to slip. Not sure why the “trigger” of the mandatory call to the CO in min CPA situation, might be a case of failed leadership.