Sleep is crucial for survival and animals have evolved different strategies to get enough rest while staying safe from predators
Walruses sleep floating on the water’s surface, and they have a built-in anti-drowning device. By inflating a sac in its neck, a walrus ensures its nostrils are safely above the water line before settling down for a snooze. Other eared seals shut down half their brain at a time and steer lazily with only one flipper. Being half-awake helps a sea lion escape a predator like a shark or killer whale.
Albatrosses are the only birds that can fall asleep while flying. To avoid the danger of landing on the ocean surface for a nap, an albatross catches snatches of sleep while gliding above the waves. Thrushes cannot do this, so they land for eight-second power naps while migrating.
The only invertebrate that sleeps in a similar way to higher animals is the octopus. Other invertebrates experience short spikes of brain activity while asleep, but octopuses and vertebrates experience slow, regular waves of activity throughout their rest. While asleep, octopuses twitch and even change colour, suggesting that they are experiencing a dream.
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Image from flickr.com/photos/hoppy1951