Book excerpt: The average human sleeps a total of twenty-four years over a lifetime. That's a lot of naps! Yet people aren't the only ones who enjoy a good rest; if you look around, you'll find that all animals have a biological need for sleep. But some animals snooze in ways that we would find startling--if not absolutely impossible. A sooty tern, an island bird, takes a nap in midair as it slowly flaps its wings. A fruit bat gets forty winks while hanging upside down from a tree branch. A bottlenose dolphin can put half of its brain to sleep while it continues to swim. What other remarkable methods of sleep exist?