[lvlrsSTI]

Actually, frigatebirds are not the only animal that sleeps half a brain at a time. It's a documented but unusual rest pattern called unihemispheric slow-wave sleep, and dolphins do it, too. A dolphin will shut down one hemisphere of its brain, leaving the other half of the brain to monitor breathing functions and keeping that eye open to monitor its environment. The left eye will be closed when the right half of the brain sleeps, and vice versa.