I was chatting with a fellow at my gym the other day. In his early twenties he spent time in a village in Africa where there was no running water nor electricity. He noticed a couple of things. First, when the sun went down... they went down. When the sun rose the following morning, these guys were stoked and ready to start the day. Hunting, fishing, and food gathering were daily activities. My friend was struck by how easily they were able to pick up stones that he struggled with. And this guy is one of the strongest dudes at my gym!
Every villager was lean - no exceptions. Somebody in my line of work would go broke. The women's menstrual cycles were in sync not only as a group, but also with the moon. I don't have a guess as to the scientific explanation for this, but it shows how in-tune with nature we are capable of being.
Our challenge is to be healthy (and happy) in a modern world far removed from the one in which we once thrived. We're slowly learning that we can't simply opt out of nature.