Russell Foster is a circadian neuroscientist: He studies the sleep cycles of the brain. And he asks: What do we know about sleep? Not a lot, it turns out, for something we do with one-third of our lives. In this talk, Foster shares three popular theories about why we sleep, busts some myths about how much sleep we need at different ages -- and hints at some bold new uses of sleep as a predictor of mental health.<br /><br />TEDTalks is a daily video podcast of the best talks and performances from the TED Conference, where the world's leading thinkers and doers give the talk of their lives in 18 minutes (or less). Look for talks on Technology, Entertainment and Design -- plus science, business, global issues, the arts and much more.<br />Find closed captions and translated subtitles in many languages at <a href="http://www.ted.com/translate" target="_blank" rel="nofollow">http://www.ted.com/translate</a><br /><br />Follow TED news on Twitter: <a href="http://www.twitter.com/tednews" target="_blank" rel="nofollow">http://www.twitter.com/tednews</a><br />Like TED on Facebook: <a href="https://www.facebook.com/TED" target="_blank" rel="nofollow">https://www.facebook.com/TED</a><br /><br />Subscribe to our channel: <a href="http://www.youtube.com/user/TEDtalksDirector" target="_blank" rel="nofollow">http://www.youtube.com/user/TEDtalksDirector</a>