Why Do Some Animals Sleep Standing Up?
Written By: Rebecca Levi
Most animals seem to have a biological need for sleep. The way they sleep, however, differs across species. For example, some animals hibernate in the winter, which involves a prolonged period of alternating between longer bouts of sleep and a state that falls between sleep and wakefulness, and others don’t.
Many animals, like dogs, cats, and humans, sleep lying down. Research has found that others, like certain species of migratory birds and fur seals, can put half their brain to sleep while the other half keeps them flying or floating. And, there are even animals that perform the impressive feat of sleeping while standing up.
What Animals Sleep Standing Up?
Particular animal species, such as Asian elephants, giraffes, and cows, have evolved to sleep while standing up. Researchers hypothesize that sleeping in an upright position may make it easier for them to wake up and get away if a predator approaches during sleep.
The bodies of mammals that can sleep while standing have developed a way to keep them upright while asleep, usually by locking a set of muscles and tendons into place. However, studies show most of these animals can only stay upright during non-rapid eye movement sleep (NREM) sleep. During rapid eye movement (REM) sleep, the body experiences a temporary loss of muscle tone, known as muscle atonia. As a result, most animals need to lie down for REM sleep.
The exception to this rule is birds. Researchers have found that certain species of birds seemingly enter REM sleep while standing. These birds might show a loss of muscle tone in their necks as they sleep and stand, but they still maintain enough muscle tone in one leg to balance and stay upright. Scientists are not quite sure how they do it.
Studies show that horses spend five to seven hours of their day asleep or resting, most of it at night. Around 80% of their sleep takes place in an upright position. They must lie down for at least 30 minutes of their daily sleep in order to obtain just under five minutes of REM sleep.
Experts have learned that horses sleep standing up by using a "stay apparatus," which involves both of their front legs plus one hind leg keeping them upright. That hind leg muscle essentially “locks” their knee into place, enabling the leg to stand upright and support most of the horse’s body weight, without requiring much energy at all. Meanwhile, the other hind leg takes a break, bending slightly with the hoof grazing the ground.
Certain species of elephants appear to be the animals who sleep the least. According to a study of two wild African elephant matriarchs, they sleep only about two hours per day, usually during the night. Zoo elephants may sleep longer, up to seven hours per day.
Elephants can sleep standing up or lying down, but, like horses, they lie down in order to enter REM sleep. In fact, they may only enter REM sleep every few days. In some cases, they may take a break from sleep altogether, and have been observed staying awake to travel for nearly two days straight.
Scientists have identified a few indicators to determine when an elephant is sleeping. When standing up, elephants close their eyes and let the end of their trunk droop to the ground. If it hasn’t moved for five minutes, it is safe to assume they are asleep.
Studies show giraffes can sleep while standing up, with eyes open and head up. However, once they enter REM sleep, they lie down, their eyes close, and their head falls to the side as their muscles relax. Giraffes sleep mostly at night, but nap during the day as well.
Early studies of zoo-based giraffes found that giraffes sleep about 4.5 hours per day, with less than 5% of total sleep time in REM sleep. More recent studies have observed giraffes lying down for much longer at night, between seven and close to nine hours. Still, only 1% to 3% of that time was spent in REM sleep. By comparison, humans spend about 25% of sleep in the REM stage.
Like elephants, giraffes are herbivores. Herbivores tend to sleep less than carnivores, and the larger the herbivore, the less they sleep.
Like horses, flamingos can sleep while standing thanks to a stay apparatus. This anatomical arrangement enables the muscles and ligaments in one leg to lock into place and keep them upright without exerting much effort. Flamingos often stand on one leg while awake and asleep, using the stay apparatus to save energy. You can tell when they are sleeping because their eyes close and they sway less.
Research shows that, on average, dairy cows sleep for four hours each day. This sleep occurs in short bursts of three to five minutes. In total, cows spend about three hours in NREM sleep, and 30 to 45 minutes in REM sleep per day. They stand up while in NREM sleep, but like other large mammals, must lie down to enter REM.
Cows spend about half their day lying down, but they’re only asleep for some of that time. Like sleep, lying appears to serve an important biological function for cows. If they are prevented from getting their lying time, they’ll act out by stomping around or even headbutting other cows.
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