I know this has been proven for high school students as well. There have been some interesting studies about starting times and standardized testing as well. Check out the study
"Examining the Impact of Later High School Start Times on the Health and Academic Performance of High School Students: A Multi-Site Study"
AFAIK the early hours classes are meant to build discipline, not to optimise for learning. A lot of school rituals seem to be built with the goal of making you used to a disciplined punctual life, rather than actually teaching you any knowledge.
My experience (as someone who was elected to public education administration) suggests it's often more about logistics. For example, adjusting how many bus drivers (and busses) are needed in a district to serve elementary, middle and high schools. Staggering start times reduces the need, but drives one of these options to be earlier than the others.
High school sports complicates logistics later in the day.
It's not just a question of learning; it's also a question of cost and resources.
