Because horses are big animals, their blood flow can be restricted by laying down for long periods of time. This causes excess pressure on their internal organs, which is why they only lay down for REM sleep. This results in them sleeping while standing up at various points throughout the day.

How long can a horse sleep standing up?

Horses may snooze for a few minutes at a time standing up and may lie down for a few hours in REM sleep. These short sleep times can add up to anywhere from a few hours to more than 12 hours per day. Younger horses as well as fillies and colts sleep longer hours than adult horses.

Do horses prefer to sleep standing up or lying down?

As they grow, they take fewer naps and prefer resting in an upright position over lying down. Adult horses mostly rest while standing up but still have to lie down to obtain the REM sleep necessary to them.