Neuroscientist Russell Foster opens a session of TEDGlobal all about... us, asking the question: Why do we sleep? Thirty-six percent of our lives are spent asleep, which means, if you live to 90, you'll have slept for 32 years. But we don't appreciate sleep enough, says Foster. He quotes Thomas Edison -- "Sleep is a criminal waste of time, inherited from our cave days" -- and Margaret Thatcher -- "Sleep is for wimps." Simply put, says Foster, not only do we not appreciate sleep, but we treat it like an illness and an enemy.