Teen falls out of bed from hanging his head off the bed in his sleep

by Kaye
(Bangkok)

My 16 year old son hangs his head off the bed while asleep to the point that he falls out of bed and has no recollection of what happened. It happens about 3-4 times per week. I think it might be a form of SPD.

I know it is the hanging off the bed that causes the problem because some days the mattress topper is hanging over the edge of the bed by 6" so I figure he must have been hanging and his weight made the mattress pad travel and eventually he hit a tipping point and fell. However, some nights he is on the floor without any shift in the mattress bad.

There is a desk covering the first 2' of his bed. I tried wedging a chair against the side of the bed, but he still falls out even though the desk and chair cover almost half the length of the bed so he must be moving around to seek the hanging sensation.

I heard the thud of his falling one night when working outside his bedroom - there was no noise like kicking or screaming before the thud so I don't think it is nightmares. He sleeps 9 hours per night in a very deep sleep.

How can I determine what is causing the hanging in his sleep and stop it permanently?