I believe that the fear of dark, once sets in, doesn't leave easily.
The toddler is 13 months old. She starts crying if left alone when the light goes off at night. She clings to me as long as the light doesn't come back.
Even if the other rooms are properly lit and she's not alone in the house, she doesn't dare to enter the kitchen if its dark.
I mentioned kitchen because it is attached to our living room. Last time when she was playing with a toy, the toy ran in the kitchen accidentally. I was sitting besides her and I told her to go in the kitchen and bring the toy. Instead, she just sat near the kitchen door and started crying while pointing towards the toy.
When I lit the kitchen she went and brought the toy out.
This toddler can run and climb on the sofas, beds, and the window grills, and neither she's afraid of heights and fast swings etc.
BTW, I am 31 years old and quite afraid of the dark.