It's a dangerous world out there, with predators always lurking. So what on earth would give every single animal in the kingdom the gumption to think it could lay itself down each day, let down its defenses, and go to sleep? Well, turns out that many species might not be as "out cold" as land mammals. We join Charles Amlaner and Steven Lima and their team at Indiana University who show us iguanas sleeping only half their brain at a time. That’s right. They sleep with one eye open. (Cue the Metallica). That way, the iguana can watch for predators, as the other half of its brain takes a rest.

While these creatures stay half awake to protect themselves, for humans, the dangers of sleep can come from within. We visit the Hennepin County Sleep Lab, where Dr. Carlos Schenck shows us some of the most terrifying sleep disorders around.