white noise

I can't see how it would damage hearing - unless very loud indeed. I've heard that it helps them fall asleep and stay asleep for longer, as it mimics sounds they'd hear in the womb.

The NHS website says all the white noise machines exceed the 50 decibels recommended for hospital nurseries. NHS states that prolonged exposure to this level of white noise can be damaging to infant hearing and auditory development.
It's probably a good idea to look at the NHS website yourself if you are concerned.

My daughter uses the white noise on her phone, so the volume is under your own control. She uses the rain noise and I like the ocean. It helps our grandson to nod off.

My DIL downloaded a free 'audiobook' so she can turn the volume down too.

Not sure about that one, if anyone has got actual research, I'd be interested.

The one that concerns me, is that habit of parents of leaving a light in at night.

light on! Lots of research showing it can lead to short-sightedness.

I hated the full dark as a child but not allowed any lights. Still do. but you can get little plug in night lights which just take the edge off the dark with a faint glow. All the GC have one.

I have tinnitus and many prefer to go to sleep with either white noise very low level or some vv quiet music. I'm sure very low level noise does no harm.

In other cultures and in our own culture where you have several people sleeping in one room and maybe on a main road with thin walls many children go to sleep in busy noisy situations.