Giraffes sleep 5 minutes at a time. For the lanky ones of the savannah, getting back on their feet is a complex operation in three steps, too long in case of attack by a predator. The solution is to rest in blocks of 5 minutes, often (not always) remaining standing, sometimes with an open eye or a guard companion, and for a total of half an hour at most per day. The giraffes in captivity are granted 4-5 hours of daytime rest instead.
Sperm whales have heavy sleep. It was thought that these cetaceans had one cerebral hemisphere resting at a time, like the dolphins. But a 2008 research on some sperm whales in Chile showed something different: mammals slept so deeply that they did not notice the imminent passage of a boat. Only when the boat hit a member of the group did the comrades leave. The suspicion is that they rest deeply, without moving or breathing, for 10-15 minutes at a time. They could also have a second type of sleep, a hemispheric like that of dolphins and migratory birds, but so far it has not been documented.
Wild ducks sleep in a row. Those at the ends keep the external eye open and watchful; those in the middle, both close them.
The walruses would sleep in any position. They have even been seen sleeping in the water, while using their tusks to hold ice floes. They can rest underwater, holding their breath for 4 or 5 minutes, but they give their best on land, where they reach a deep sleep that can continue for 19 hours.
Why do bats sleep upside down? Because their wings would not produce enough thrust to take off from the ground and from the ground. These creatures therefore use gravity to start their flight, and at the same time keep themselves away from the ground and from dangers. Thus they spend an inordinate number of hours: on average 19.9, a record that makes them among the most sleepyheads in the animal world.
The world upside down bats
The otters are hooked so as not to drift. When they rest on their backs, in order not to be carried away by the currents, the sea otters (Enhydra lutris) hold each other to form small floating groups. Alternatively, they can be held with the legs to the surface algae, using them as anchors.
The frogs sleep covered up to the nose. Those that face summering - a sort of summer lethargy characteristic of the arid areas of Africa and South America - generate a "sheet-bag", losing several layers of skin. They wrap themselves in this cocoon leaving only the snout to breathe, and re-emerge at the first rains. Frogs that face hibernation partially cover themselves with mud (if aquatic) or earth (if terrestrial), but not without activating a special antifreeze. As soon as ice crystals form in their body, a massive dose of glucose protects the internal organs from freezing.
The most beautiful pictures of frogs
Sharks sleep on the go. Except for a few exceptions, which concern smaller species, these predators also swim in their sleep. Otherwise, they could not breathe: to do so they need the water to continue to pass through the gills. In 2016 a great white shark was filmed for the first time while he slept slowly swimming in shallow waters, with the jaws open in the direction of oxygen-rich currents (below, the video).
Sleeping in a bed is a record-breaking habit. Also like orangutans, gorillas and chimpanzees. The latter meticulously reconstruct their own bed of leaves every night, in a pre-nocturnal ritual that is so similar to our tooth-washing, reading and glass of water.
Because everything (or almost) what you thought you knew about sleep is wrong