The child doesn't have to be afraid of the actual black darkness, but they are afraid waking up when they can't see a fucking thing, they can't see their familiar room, their bed, their blanket, toys etc, another human?
It is natural for babies and children to wake and seek comfort, I dread to think of all these trained babies and kids in another 20 years and see how this treatment affects their relationships. I've said it before and I'll say it again, leaving your baby that's looking for your comfort, crying or not, is abuse. It's neglecting their needs. If you want a full night's sleep, get a baby born, do not have kids