Russell Foster studies sleep. In fact, he's a circadian neuroscientist, which means he studies what happens to the brain when it does - and doesn't - sleep. This entertaining talk, just under twenty-two minutes in length, covers a range of sleep-related topics from the amount of sleep people typically got before the invention of the lightbulb (hint: a lot more than we get now) to dangers of depriving teenagers of their bed rest. Along the way Foster asks, and tries to answer, the age old question of why we sleep, and explores the relationship of interrupted sleep cycles with mental illness. Subtitles for the talk are available in 36 languages.