Here's a five-minute TED Talk in which Jessa Gamble talks briefly about how animals (including us) have natural sleep cycles. What happens when you take away a person's watch and shut him in a bunker, deep underground, for a period of months? What is a "natural" sleep cycle for humans, and how does it differ from typical sleep cycles that we practice?
If you enjoyed that tiny tidbit, check out Gamble's book Siesta and the Midnight Sun: How We Measure and Experience Time (currently available only in Canada).
In this tangentially related talk, Arianna Huffington argues for more sleep (including the memorable quote "We've had far too many icebergs hitting our Titanics"):