A.M.
I have a friend with a 4 year old daughter that does the same thing, she even bangs her head on her carseat when she is tired in the car. Her daughter has been doing it for years. When my friend asked her doctor about it, he said many children actually do this, it is a soothing technique for them. He said as long as they aren't hitting their heads so hard that there is swelling or bleeding then they are fine. Some children like to be rocked, some like to suck on something and some like to bang their heads.