Horses certainly can and do sleep standing, but at some point, all horses need to lay down in order to get the full sleep cycle in place and to avoid sleep deprivation. In fact, many horses choose to sleep lying down rather than standing up, simply because it is far more comfortable. Most horses will lay down to get some deeper sleep several times each night, as long as they have a comfortable area in which to do it, and they feel safe doing it. Horses will lie down quite frequently to sleep deeply, often during times when there is not much else going on.
While we have to lay down to get a good nights rest, horses can sleep standing and lying, though they enjoy their REM (rapid eye movement) sleep, which is very deep sleep, only when lying down. Horses can get some amount of rest when standing, but in order to get to REM sleep, which is deep sleep, also called paradoxical or desynchronous sleep, they need to be lying down. Adult horses mainly rest when standing, but must still lie down to get the necessary REM sleep. The minimal time horses spend lying down for REM sleep shows horses ability to gain needed rest when standing.
Horses can sleep a few minutes standing and can lay down for hours at REM sleep. When horses require deeper sleep, however, they lay down, typically for a number of brief periods, totaling approximately two or three hours per day. Horses require 30 minutes to three hours of REM sleep per day, but this accounts for a very small portion of their resting habits. Horses might not have to lie down to catch REM sleep, they really need to spread out to their sides in short 10-to-20-minute bursts during the evening to get this REM sleep.
While horses may take a short nap when standing, they cannot get the REM sleep they so desperately need without relaxing all of their muscles. Horses can doze while standing alone, just like us humans might on a couch, but in order to get a proper restful (REM) sleep, they need to lay down. Although horses in stalls can also snooze standing, horses clearly must lay down to rest and sleep, at least part of the time. Horses may get a brief nap standing, feed a bit, then sprawl on their sides for a few minutes of deeper sleep.
You might not see them lying down a lot, since they mostly get their hard sleeps after midnight during dark hours of the night. Interestingly, if you watch a herd of horses in the field, you will often see them all lying down asleep except for one. Sometimes, in a stable with ten horses, you can see nine lying down in a stall, asleep, and a single horse staying up and alert. I seldom see our horses sleeping; indeed, I am rarely surprised to catch a glimpse of our horses standing still for long periods of time in a pasture; usually, they are walking around and eating.
Your horse may nap briefly when standing, and often does, particularly if it is boring, but he will also lay down for better sleep at night. During slow-wave and rapid-eye-movement (REM) sleep, a horse must lie down, as a horses muscles, joints, tendons, and ligaments will be in a state of weakening (atonia) that cannot be supported by any standing device. REM sleep produces jerky eye movements, accompanied by chaotic, fast brain waves, and occurs mostly with the horse lying stretched flat on its sides, not backed against its chest.
If the horse is not getting sufficient REM sleep, then the horse can collapse due to a failure in the Equine Stability Device when they enter rem-sleep when standing, causing serious physical injuries. Overweight horses also may have difficulty lying down and rising from lying, which may limit the amount of REM sleep.
If a horse does not get adequate amounts of sleep, adverse effects may not become apparent for several days, but the horse can ultimately become irritable, have poor temperament, or even be dangerous. Common symptoms of horses who are getting insufficient sleep include excessive drowsiness during the day, bruising on knees and fetlocks (from collapsing episodes), unwillingness or inability to lay, and impaired athletic performance. Horses who are not able to get a good nights rest over the course of weeks and sometimes months suffer from it with regard to their physical performance.
Horses will sleep at different times throughout the day, and will experience brief periods of deep sleep lying on their backs during the night. When standing, horses fall into deep dozes, but they can wake and be alert very quickly.
To enter true deep sleep, all the skeletal muscles need to relax; relaxation cannot occur while a maturing horse is standing. Some animals which are capable of standing to sleep, like horses, cows, and elephants, lay down while they are in deep REM sleep in order to prevent collapsing while experiencing muscular atonia. Just as humans feel this falling sensation, jolting up occasionally, horses twitch and wiggle during their sleep.
Horses do snooze while standing, but get the deepest slumber like you would when you are lying down and relaxing–explaining why and how they do it is trickier. In a 24 hour period, horses need at least 30 minutes in recumbency to meet their need for deep sleep. Sleep deprivation, colic, and other diseases may play a role if the horse is spending significant amounts of time lying during the daylight hours.