Horses sleep roughly 2.5–5 hours per day, and their sleep is divided into rapid eye movement (REM) sleep and deep sleep. In addition, horses rest and doze on their feet. There is little information available on equine sleep. It has been assumed that it is necessary for horses to lie down to sleep properly, as they are unable to enter REM sleep while standing. The sleep of horses is easily disrupted by changes in the environment, for example, in connection with migration to pasture. In exceptional circumstances, horses can postpone lying down for as long as six weeks. It is not known what effects delays in sleep and rest have on horses. The ability of horses to postpone sleep is exceptional among animals.