Russell Foster is a circadian neuroscientist which means 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.