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Sleeping while standing is even more difficult for humans than sleeping sitting up due to the loss of muscle tone that occurs during REM sleep. However, this practice has been observed in certain situations, such as soldiers on nighttime sentry duty.
Other animals are more adept at sleeping on their feet. Horses, for instance, are equipped with a system of tendons and ligaments that allow their legs to essentially lock into place while they sleep. Flamingos are another example, and can actually sleep while standing on one leg.
It’s important to note that some sleep disorders can cause people to stand up, and even walk around, without waking up. Sleepwalking is a disorder of arousal. During sleepwalking episodes, a person may get up and walk or run away from their bed. This behavior may occur in people who experience sleep terrors, another disorder of arousal. However, this should not be seen as normal or healthy sleep since it is disorder-related.
