[saulrh]

Grades in primary and secondary school jump significantly if you push start times back from 8 AM to more like 10 AM. It may be the biological default to wake up in the morning, but there's absolutely nothing that says that peak performance should be reached the instant you wake up. If anything, I'd expect exactly the opposite! It takes time to get caches warmed, adjust biochemistry away from resting levels and toward operational levels, and get dumb maintenance tasks (breakfast, travel to workplace, "milk the cows") out of the way and move to higher-value and more complex activities (hunting, crafting, socialization). Until you do those things you're not going to be operating well, and many early-morning tasks wouldn't require peak performance and optimizations for an early peak would be wasted. I'd also expect intellectual pursuits to be more strongly affected; you need to be able to not get eaten by a tiger at absolutely any time, but you can choose when to sit down and start knapping flint or talking to people, which means you have more freedom and reward when optimizing the timing of peak performance on intellectual tasks.