Allāhumma qini ‘adhābaka yawma tajma‘u or tab‘athu ‘ibādak (O Allah, protect me from Your punishment on the Day You gather or resurrect Your servants)

The merit of this blessed supplication and the recommendation of reciting it frequently, following the example of the Prophet (may Allah's peace and blessings be upon him).
Lying down on the right side is recommended.
As-Sindi said: His statement (O Allah, protect me from Your punishment) implies that a wise person should treat sleep as a reminder of death and the resurrection that follows it.
Protection from Allah's punishment on the Day of Judgment is through His grace and mercy, by guiding the servant to righteous deeds and by Allah's forgiveness of his sins.
Humility of the Prophet (may Allah's peace and blessings be upon him) before his Almighty Lord and Master.
Affirmation of resurrection and the return, and that people will be brought back to their Lord to be held accountable for their deeds. Whoever finds good should praise Allah Almighty, and whoever finds otherwise should blame none but himself, for they are merely the deeds of the servants that Allah keeps count of against them.
Keenness of the Companions (may Allah be pleased with them) to elaborate on the conditions of the Prophet (may Allah's peace and blessings be upon him) in his sleep.
His statement: "He would place his right hand under his cheek": this was from the Prophet's habit of favoring the right side in all matters, except in cases where there is evidence indicating otherwise.
Sleeping on the right side leads to quicker wakefulness, as the heart is less settled in that position, and it is more restful for the heart because it is located on the left side. If a person sleeps on the left side, it may harm the heart due to the body's weight pressing upon it.