Their muscles are in a relaxed state.
This phenomenon has mainly been studied in horses. It was found that there is a mechanism in their knee joint that locks it in a straight position, allowing the horse to remain on its feet without muscle tension.
During sleep, the horse periodically changes position, shifting from one leg to another (possibly without waking up) to prevent blood stagnation in the veins and avoid swelling of the limbs. If possible, horses prefer to sleep lying down, and in this position, they experience phases of rapid sleep (dreaming), when the muscles of the body are completely relaxed.
