Not sure about you, but most people find it rather difficult to sleep standing up. The closest humans get is sleeping in a chair or the astronauts in the International Space Station who sleep in bags stuck to the walls. While humans prefer to sleep lying down there are a number of mammals that can sleep standing up.
Most hoofed herbivores—horses, cows, camels etc.—are able to sleep standing up. Part of the reason that they do this is that it is important for their digestion processes. These hoofed herbivores need to break down cellulose which is a notoriously tough material and the digestion process takes a long time. Standing up while asleep allows their stomachs to continue this difficult process during the night.
While these hoofed herbivores can sleep standing, they do have to lie down when they are in REM sleep. This is because REM sleep causes muscular antony and they would fall over.
This ability to sleep standing up has lead to the propagation of a myth about cow tipping, where people supposedly sneak up to cows while they are asleep and push them over. All experiments show that while in the non-REM sleep the cow is almost impossible to sneak up on and equally difficult to push over.
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