Normal adult horses don't sleep like people. They only average three to five hours of sleep each day. The sleep occurs in five to seven phases of 30-40 minutes each. SWS always precedes REM sleep in normal horses. Less than one hour per day is spent in REM sleep.
Horses normally go to sleep standing up.
At this time, the horse normally lies down on its chest. SWS primarily occurs when the horse is resting on its chest. In REM sleep, the horse lies on its side. The horse cannot complete a sleep cycle without lying down. Narcolepsy is characterized by the sudden onset of unexpected REM sleep.