The sleeping habits of horses are somewhat different than humans. Most human sleep is usually a long stretch – about eight hours in a 24-hour day. Horses rest for short periods during the day more frequently. They are not strictly nocturnal or diurnal, like most humans.
Also, horse’s sleeping patterns change depending on their age.
Baby horses lie down for frequent naps and spend about half of their day sleeping until they are about three months old. As the horse gets older, the frequency of the naps becomes less, and they are more likely to stand rather than lie down since mature horses most frequently rest in a standing position because lying down is actually more stressful for a horse than standing as their own weight causes pressure on their internal organs.
Some horses do sleep stretched flat out on the ground and may even snore and twitch as dreaming. This may have been equivalent to the REM phase of sleep in humans. Of course, apparent snoring may be caused by the increased weight on the respiratory system.
Horses will lie down to sun themselves, and it is not unusual for several horse to lie down at the same time for a communal sun bath on a warm spring day. Often while several lay down, one or two others will remain standing as if on watch. Horses tend to spend less time lying down in cold snowy conditions, although on a sunny day, some will snooze stretched out in the snow.http://estanciadayargentina
Each horse has its own sleeping habits. Some will sleep only at night while others sleep during the day as well.