Sheikh Yûsuf `Abd Allah Ahmad, professor at al-Imâm University
The Prophet (peace be upon him) used to sleep on his right side. He also instructed his followers to do so.
He said: “If you come to your sleeping place, perform your ablutions as you would for prayer, then lay down on your right side, then say: ‘O Allah, I have submitted myself to You and have entrusted to You my affairs, and I have taken refuge in You, in both hope and fear of You. There is no refuge or sanctuary from You except with You. O Allah! I believe in Your Book that You have revealed and Your Prophet whom You have sent.’ If you die on this night of yours, then you will be on the natural faith. Make these words the last thing that you utter.” [Sahîh al-Bukhârî and Sahîh Muslim]
We know that there must be wisdom for laying down upon our right sides when we go to sleep. However, since nothing has been mentioned to us about it in the sacred texts, we cannot say what this wisdom is with any degree of certainty, except to say that we hear and we obey.
And Allah knows best.
Source: Islam Today