Sleep on the back is that of the prophets, reflecting on the heavens and the earth; sleep on the right side is the sleep of the scholars and worshipers; sleep on the left side is that of the kings—who need to digest their food; and sleep on the stomachs is that of the devils.
Abū Ḥāmid al-Ghazālī [d 505H/1111CE], Iḥyā ‘Ulūm al-Dīn