Sleep
Our Hunter-Gatherer Ancestors weren't pressured to be at work for a set time, they went to bed when tired and woke with the dawn. Ancestral Sleep is about prioritising sleep, having as many great night sleeps as you can and to wake up naturally.
Back a few thousand years, sleeping for 8 hours a day from a survival point of view doesn't seem like the best thing to do, as we're vulnerable for 7-8 hours. However, evolution hasn't selected to remove sleep, which means it is more critical to our survival than being vulnerable while we sleep.
We now know that sleep is when the body cleans the toxins from the brain, and is critically important for overall health, performance, and longevity. Typically we need between 7-8 hours of quality sleep each night.