I spoke to his doctor and researched this behavior online. All sources said that this is a way for toddlers to self-soothe. Something about the repetition (and pain?) is somehow soothing to them. I was told not to worry about it and that it peaks around 18 months of age.
Well, it's been hard not to worry about this behavior. Or ignore it. Particularly in the middle of the night when all I can hear is the SMACK SMACK SMACK of my baby's head against solid wood.
We finally got smart last weekend and decided to add more padding to the crib. We dug up Ryan's old crib bumpers and tied them to the crib on top of Lucas' existing bumpers.
It's like a little padded cell now. Lucas is still banging his head all hours of the night, but it sounds much less painful and dangerous to him. I still hope this stops soon!