I hear about these things. But I have doubt based on personal experience. When I've done long hikes (did both the Appalachian and Pacific Crest Trails), I would nearly universally knock out pretty much as soon as the sun set. This, after a lifetime of being a night owl who very easily would stay up until 2am every night.

And this was true of basically everyone else I met in every age group as well. 17 year olds to 70 year olds all abserved hikers midnight, hitting their sleeping bags hard not long after sunset. Even with a campfire going and socializing to be had, staying up 2 hours past sunset was a hefty proposition.

Also, with the 2 sleeps thing - on the trail, I typically slept straight through the night. I might wake up once or twice to pee, but then I would lay back down and be right back to sleep.

My guess is that

1. All humans naturally sleep around sunset and wake at sunrise, with negotiable dispositions to sleep earlier or later based on age.
2. Variations in sleep schedule in ancestral environments would account for disposition, but would likely be driven by cultural norms, not instinct.
3. Having two sleeps was probably an artifact of having easily available illumination (candles and lamps) which occurred at this time.