The book "Why We Sleep" talks about how all animals (and plants) have circadian cycles, and the low point of the cycle is marked by a low body temperature. They

The book "Why We Sleep" talks about how all animals (and plants) have circadian cycles, and the low point of the cycle is marked by a low body temperature. They have done cross cultural studies and can see that the circadian cycles of teenagers are later, and that the cycles change throughout life. Regardless of your sleeping habit the circadian cycle seems to hit it's low temperature point at the same time. Thus, if you start going to bed at 8 PM every night your low body temperature will still arrive at the same time, regardless of your sleep habits. Take this all with a grain of salt, because my memory of the book is not infallible. It might give you a place to start researching how much of our sleep behavior is innate vs environmental as you ask. It's certainly not a new idea to sleep researchers, I bet there is an answer out there. As for the "Why We Sleep" book. I found it well written and it is well reviewed by many, so you might be interested in it.