I think it depends on her age. If she is under 5 or 7, I can try to turn on a night light beside her bed, so she will get used to shady places. and with time you could make the light weaker and weaker. Or you could make it a game, every time when you would make the lighter weake, you could tell her that she's got to the new level of the game. She will win it, after being able to go to sleep without any light.
If she is older, I think there is no need to do something special. She just has to overcome it by herself.
