Sleep has to be essential to life as we know it, because no animal has completely outevolved it. Some animals sleep an hemisphere at a time (dolphins, long-flig

Sleep has to be essential to life as we know it, because no animal has completely outevolved it. Some animals sleep an hemisphere at a time (dolphins, long-flight birds) or in fragments (giraffes), but none truly goes without sleeping no matter how challenging their environment is for a nap.