In series ½/3, whenever we see Morgana sleeping she lies on her back, hands resting on her chest. She’s open and trusting.
From season 4 she sleeps on her side, hands beneath chin, curled up. Morgana didn’t just become evil, she became frightened. She lost any illusion of protection and became used to protecting herself. Her own body became her only comfort and she no longer trusts the world not to harm her while she rests.