Video: Jessa Gamble: Our natural sleep cycle FROM TED Talk [4 minutes]

"Well, it turns out that, when people are living without any sort of artificial light at all, they sleep twice every night. They go to bed around 8:00 pm. until midnight and then again, they sleep from about 2:00 am until sunrise. And in-between, they have a couple of hours of sort of meditative quiet in bed. And during this time, there's a surge of prolactin, the likes of which a modern day never sees."