That’s a good diet so I’m going to rule that out, with the exception of adding a good multi vitamin into her diet. Smartpak has a good one “SmartVite”, and there is also Mega Cell, and Red Cell, that you can get almost anywhere. I would add that to her mix to see if she get’s a boost out of it. Some horses, like people, rest easier and require more naps. Considering her age, she may just need more rest than most, and it’s a wonderful thing to think she is so at ease with her environment, that she actually lays down, and enjoys a good nap. If she still seems off to you, though, even with the vitamins added, and you are still worried about her, I would contact your veterinarian, to rule anything bad out. Good luck, please keep me posted!!