Posting on behalf of a friend:
My friend has a dd of 2 yrs, 8m. She has been head banging to a varying degree for a year.
She does it whilst asleep, banging the back of her head against the head board of her bed. It's very forceful and loud. Her parents will go in, lie her down, leave, numerous times a night.
She's seen the doc/a specialist at hospital, who discharged her after checking her development.
Strangely, she doesn't always do it when sleeping away from home.
Anyone had experience of this?