Stabling and turnout practices are dependent upon many factors. These include owner preferences, setup of the barn, number of horses, size of paddocks, availability of outdoor shelter, sources of food and water, gender mix, as well as the health and work requirements of individual horses.
It seems to me that a lot of horses are kept in stalls most of the time, with many stables limiting turnout to a fraction of the day and keeping horses inside during the night and in cold weather. This makes sense based on our human schedules and needs. But does it make sense when considering the different sleeping and eating schedules between humans and horses?