There is no specific length of time that a horse can lie down. However, the longer a horse lies down, the greater the risk of injury, according to the University of Illinois at Urbana-Champaign College of Veterinary Medicine.
Because horses are large animals with considerable body weight, lying down for too long restricts blood flow to certain areas. Restricted blood flow often causes problems when the animal attempts to stand again as blood flow attempts to normalize. One injury that can result from restricted blood flow is a reperfusion injury.
Other problems that can occur as a result of a horse lying down for too long include injuries to muscles and nerves due to the pressure on them or blood pooling in a lung. When horses go into surgery, veterinarians recognize that they only have a matter of hours to keep a horse in a lying position without causing the animal harm.