Like virtually every other animal, humans need sleep to survive. But why? What do we gain from spending nearly a third of our lives in such a defenseless state? And what's at stake if we sleep poorly? The functions of sleep have long been something of a mystery. But now scientists are peering deep into the brain to see what happens while we snooze. One study reveals that well-timed bursts of pink noise can nudge a person's brain waves, extending periods of slow-wave, or deep, sleep. And toddlers learning new words show that taking even a short nap after learning something new can help the information stick. And when it comes to sleeping poorly, it may turn out that having interrupted sleep can be just as harmful as missing sleep altogether. As scientists come to understand the powerful role that sleep -- or lack of it -- plays in everything from memory to trauma to emotion regulation, one thing is clear: This biological function is as crucial as it is complex.