Did You Know?
Horses with severe sleep deprivation can, in very rare cases, fall down on their own, even if they are running at the time. This occurs because they unconsciously slip into the much-needed REM sleep.
Humans have evolved to sleep for one large portion of the day. Protected from the primal dangers faced by cavemen, we feel sluggish if we sleep for anything less than the traditionally recommended 8 hours.
Most animals, on the other hand, do not follow this pattern of sleeping. Most prey animals have evolved to be able to survive with much shorter bouts of sleep. Horses, prey animals in the wild, have evolved likewise. In the wild, a prostrate animal is prone to predation, and prey animals have evolved to be able to sleep for short bursts, and be ready to flee if a threat arrives.
Unlike humans, horses don't need a continuous stretch of sleep. They sleep in scattered patches, and only need about 2-2.5 hours of sleep during a 24-hour cycle. They usually only sleep for a few minutes at once―usually less than 15 minutes―accumulating their total daily sleep from several such periods of inactivity.
Horses also rest their legs by merely standing without sleeping. The time spent awake but merely standing is much longer than that spent sleeping.
How do they not fall over?
Even though horses sleep while standing, they don't fall over. Not fair, right? If we fall asleep on a lovely summer afternoon in high school, our drooping heads become the target of a very stern gaze. Bet horses never get caught dozing off in their cubicle!
Horses have a unique arrangement in their feet that allows them to lock their limbs in place. This is called the stay apparatus, and is used to rest their limbs while the horse dozes off. This provides them with an excellent advantage in the wild, where they don't have to spend time lumbering up to their feet if a predator suddenly sprang from the grass.
Another trait observed in horses as a prey animal is that they usually try to face an open area while dozing off without lying down. Stabled horses usually try to follow this behavior by facing the gate of the stable. This habit allows them to make a break if a predator suddenly emerges from the undergrowth.
This may seem trivial, but a horse lying down with its head away from an escape route is almost certain to be preyed upon in the wild, whereas one standing up while facing the route to safety is much more likely to live to neigh the tale.
Do they need to lie down at all?
Yes, even horses, arguably the animals most adapted to running, need to lie down once in a while. Horses can attain short wave sleep standing up, but they need to lie down to enter rapid eye movement (REM) sleep. Horses must sleep lying down for a few hours every week. They usually lie down for about an hour or two at once. If not allowed to lie down at all, they become sleep-deprived, even though the standing sleep continues unhindered. Foals spend much more time than adults lying down, and the amount of required REM sleep progressively reduces as the horse grows older.
Horses are famously gregarious animals, and display another crucial evolutionary trait while sleeping. Horses are adapted to sleep in groups, so some members can keep a lookout while others sleep. Domesticated horses instinctively look to follow this behavior. If kept alone, horses can become very distressed, since their brain rebels against the potentially fatal idea of sleeping without the protection of an awake pair of eyes. As a result, horses are much more relaxed and sleep better when in a group.
Horses need their daily allowances of sleep just as we do. They are just much more efficient with their sleep than their oft-pampered owners. Horses can make up their daily sleep in patches, but must be allowed to lie down every few days (at least) to prevent sleep deprivation.