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Split Rock Mountain

Horses sleep standing up some of the time, but not all of the time

Horses need to sleep, the same as humans do, although they sleep about half the time humans do and in shorter bouts. But just the same as humans, horses experience - and require - three different levels of sleep. The first is light sleep, the second is slow wave or deep sleep and the third is rapid eye movement (REM) sleep.

Horses can achieve the first and second levels of lighter sleep while standing up, as in the photo of the three ponies above. They do this by locking their major joints so they do not fall over.

However, to achieve the third and deepest level of sleep, REM sleep, horses must lie down, as this level of sleep is characterized by total relaxation of their muscles. In REM sleep, horses may lie down completely flat. Or they may rest on their chest, (referred to as sternal recumbency) with their muzzle on the ground, supporting their head, as in the photo of Spok, on the Facebook page.

It is essential that horses have appropriate environments and sufficient opportunities to achieve REM sleep. If horses cannot lie down, they become sleep deprived. They may then slip unavoidably into REM sleep while still standing, upon which their legs buckle and they fall, usually suddenly, onto their knees. This can result in serious injury.

To be able to achieve REM sleep, horses need a sense of security, such as provided by a familiar, calm environment, with friends in sight. They also need sufficient space to feel that they have enough space to safely lie down and get up again.

In a group of horses, it is usual for some individuals to lie down, in REM sleep, while others remain standing, keeping guard. Horses lying down are vulnerable, not only because they may be deeply asleep and hence unaware of their environment but also because it takes them several seconds to get to their feet and be ready to flee if necessary.