"The Secret World Of Animal Sleep" [1]
"Every animal with a brain needs sleep — and even a few without a brain do, too. Humans sleep, birds sleep, whales sleep and even jellyfish sleep.
Sleep is universal "even though it's actually very risky," said Paul-Antoine Libourel, a researcher at the Neuroscience Research Center of Lyon in France.
When animals nod off, they're most vulnerable to sneaky predators. But despite the risks, the need for sleep is so strong that no creature can skip it altogether, even when it's highly inconvenient.
Animals that navigate extreme conditions and environments have evolved to sleep in extreme ways — for example, stealing seconds at a time during around-the-clock parenting, getting winks on the wing during long migrations and even dozing while swimming."