Horses are able to sleep both standing and lying down. Horses can get a light foot sleep because they can block the stifle joint without having to hold their weight with their muscles, although they need to lie down to achieve deep sleep, REM status. Horses sleep better when they are in a group because of their natural tendency to take turns and there is always someone standing guard in case a predator appears. A horse that is alone will not sleep well because his instinct makes him stay alert to any possible danger.
Unlike humans, horses do not sleep continuously, but by dividing the sleep period into small blocks. The horses rest for four to fifteen hours a day, although they will only lie down for a few minutes several times during that period. The total sleep time over a 24-hour period can range from several minutes to a couple of hours, most of them in short intervals of about 15 minutes each, although they only need to lie down for an hour or two every few days to meet their Minimal REM sleep. If a horse is not allowed to lie down, after several days it will show symptoms of sleep, and in some rare cases it may fall while involuntarily entering REM sleep while standing.