I've read this before too, I cant remember where. Basically your 25-50 year old typically sleeps from about midnight to 8am. Teens and young adults typically sl

I've read this before too, I cant remember where. Basically your 25-50 year old typically sleeps from about midnight to 8am. Teens and young adults typically sleep much later, with the "crack of dawn" not being uncommon and the elderly typically awake at the "crack of dawn". Basically most preditors are active either just after dark, or in the late morning. Teens are typically strong (a 13 year old male is as strong as your average adult female), alert and they have the best vision, and noise making in the night would keep predators and scavengers away. Evening predators likely weren't a great risk, but scavengers definitely would be an issue. Adults are obviously the strongest and most alert, and are awake when the most dangerous animals are also awake: lions, tigers, hippos, etc. The elderly are our weakest, their vision is poor and their alertness becomes questionable. However, there's theories we live so long after infertility sets in specifically for the reason they care for the young. Both infants and the elderly have surprisingly similar sleeping patterns. Predators are least active in the early mornings and so are most dangers. Insects and reptiles (venomous things) are least active in the coldest hours of the day, which is right around dawn.