By Dr. Melody Campbell, D.V.M January 2022
Did you know that horses can sleep standing up??!
It is a remarkable feat that they can do because of the way their front and hind legs are built. Horses are prey animals and they need to be able to start running in an instant if there is a threat, such as a tiger in the bushes! It takes too long for them to get to their feet when they have been lying down. For survival, they have been blessed with the ability to sleep standing up!
Horses have what is called a stay apparatus. This consists of ligaments, tendons and muscles that are designed to stabilize the joints in the limbs in a standing position. The stay apparatus allows horses to sleep standing up with a minimum of muscular activity. They use the stay apparatus with three of their limbs and rest the fourth limb. In addition, in the hind limb, there is a connection between the stifle (knee) and the hock called the reciprocal apparatus. When the stifle is locked into position, the hock is also in a fixed position. Horses that are in a standing position have the advantage of a quick escape if threatened. Also, in a standing position they avoid the cardiorespiratory compromise that happens when they are lying down.
Four resting states have been identified in horses. They are idling, resting, drowsing and sleeping. Idling is a passive waiting between activities. Resting occurs in a standing position or when lying down. A horse that is drowsing stands with its eyelids partly open and its head hanging partway down. This is the only form of sleep that occurs when standing, and is a slow-wave sleep (SWS) that allows some muscular tone to be retained. Sleeping occurs when horses are lying down and this is when they experience rapid eye movement or REM sleep.
Most horses sleep between 8:00 pm and 5:00 am, but they also often sleep in the first two hours after midday. Human activities and demands often interfere with these natural body rhythms and deny horses their afternoon naps!!
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