Easy. The decision is made for you by the RFM. If you don't have a clear night with ample celestial illumination to see the ground
But you won't know that until you get in the air and look down - it will also change with the moon position and fullness.
Just because you can look up and see the stars doesn't mean you can look down and see the ground.
In 40 years of flying I have only seen a few nights, some but not many, where you could honestly say you could fly in an unstabilised helicopter with sole reference to external cues and not need to look in at the instruments.