A loss of bearings

Fear of the dark normally occurs because you lose your bearings.
Naturally, fear of the dark expresses other fears that a child may have.
In darkness, their usual reference points disappear and the bedrooom, that reassuring place, becomes strange and frightening.
The dressing gown hung up on the door turns into a witch, toy cars become goblins, etc.
If your child tries to get up, he can't find the way to the door or light switch. 
This creates a fear of turning off the light.