Regular bedtime and test scores for 4-year-olds

This study says 4-year-olds who have a regular bedtime are developmentally advanced in "receptive and expressive language, phonological awareness, literacy, and early math abilities" compared with kids who do not. Apparently this was more important than the amount of sleep.

The study looked at sleep patterns and results of an abbreviated developmental test for 8000 kids in the Early Childhood Longitudinal Study.

I'm not sure this shows a causal relation between sleep times and development. I wonder if living in a household that has rules does it. I wonder if the kids with regular bedtimes also had regular family mealtimes, the conversation at which has been shown to help kids develop. 

And also, people vary. The fact that there is a statistical relationship between regular bedtime and development doesn't mean it's true for every person. My son has been the last to sleep all but probably 50 days in his entire life. I hate to think how smart he would be if he got to bed at 10.