As a child, you probably learned the story of Goldilocks. When she stumbled into the three bears' house and discovered their breakfast on the table, the first bowl of porridge was too hot, the second was too cold, but the third was just right. She gobbled it up. 

This is, of course, a classic fairytale. It is also a tale that apparently captures an essential truth about about how to attain peak human performance. Like Goldilocks at breakfast, our brains prefer to run not too hot, not too cold, but just right.