Short answer
The sunnah is to lie down on your right side, with your right hand under your cheek, and say the dua before sleeping. Turning onto your left side or back later in the night is fine and not makruh. Sleeping on your stomach is disliked.
Lying down to sleep on the right side is sunnah (mustahabb), preferably facing the qibla. Sleeping on the back or left side is permitted, and sleeping face down is makruh.
The four schools agree on this.
Explanation
The Prophet ﷺ told al-Bara ibn Azib: when you go to your bed, make wudu as you do for prayer, then lie on your right side, and say the dua he taught. Hudhayfah described that when he lay down at night he would put his hand under his cheek and say: O Allah, in Your name I die and I live.
So the sunnah of going to sleep is:
- make wudu before bed;
- dust off the bed;
- lie on your right side, with your right hand under your right cheek, ideally facing the qibla;
- read the adhkar of sleep, such as Ayat al-Kursi and the last three surahs, and the dua before sleeping.
You do not have to stay on your right side all night. Turning over in sleep is natural, and lying on your back or left side is permitted. The Prophet ﷺ was seen lying on his back in the mosque. If a doctor advises the left side, for pregnancy, reflux or another reason, follow that and you lose nothing: you can begin on the right for the sunnah and then turn.
The one position he disliked is lying on the stomach. See is it haram to sleep on your stomach. The four schools agree on these sunnahs. For more, see Sunnahs of daily life and when is it makruh to sleep.
The evidence
Quran, Surah Al-Furqaan 25:47
وَهُوَ ٱلَّذِى جَعَلَ لَكُمُ ٱلَّيْلَ لِبَاسًۭا وَٱلنَّوْمَ سُبَاتًۭا وَجَعَلَ ٱلنَّهَارَ نُشُورًۭا ﴿٤٧﴾
And it is He who has made the night for you as clothing and sleep [a means for] rest and has made the day a resurrection.
Sleep is a blessing from Allah, and the sunnah shows how to receive it.
Sahih al-Bukhari 247
Sahih... ثم اضطجع على شقك الأيمن ...
"Then lie down on your right side."
Sleeping on the right side is the sunnah.
Sahih al-Bukhari 6314
Sahih... كان النبي إذا أخذ مضجعه من الليل وضع يده تحت خده ...
"When the Prophet ﷺ lay down at night, he placed his hand under his cheek."
Shows the posture he used.
Where the ruling is recorded
- Riyad al-Salihin (al-Nawawi), Chapter on what to say when going to bed. Gathers the hadith on lying on the right side and the adhkar of sleep.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.