Yes my Millie does. one time I woke up and her eyes were half open but you could only see the whites of her eyes and she wouldn't wake up for a couple of seconds but when she did she just looked annoyed at me for disturbing her lol. Millie and my parents Yorkie Bonnie sleep on their backs all the time. When they're really deeply sleeping and relaxed they like to stretch out.
Love and light,
Chrissy xxx