One should recite the following Duaa when turning from side to side while sleeping at night:
لَا إِلَهَ إِلَّا اللَّهُ الْوَاحِدُ الْقَهَّارُ، رَبُّ السَّمَاوَاتِ وَالْأَرْضِ وَمَا بَيْنَهُمَا الْعَزِيزُ الْغَفَّارُ
There is none worthy of worship besides Allah, the One who is Alone (in His Being and attributes), the Conqueror, the Lord of the heavens and the earth and all that is between them, the All-Mighty, the All-Forgiving
عن عائشة، قالت: كان رسول الله صلى الله عليه وسلم إذا تضور من الليل قال: لا إله إلا الله الواحد القهار، رب السماوات والأرض وما بينهما العزيز الغفار (صحيح ابن حبان رقم 5530, سنن النسائى الكبرى رقم 7641)
Hazrat Aaisha (Radhiyallahu Anha) reports that Rasulullah (Sallallahu Alaihi Wasallam) would recite the following Duaa when he would change his position and turn from one side to the other while sleeping at night
لَا إِلَهَ إِلَّا اللَّهُ الْوَاحِدُ الْقَهَّارُ، رَبُّ السَّمَاوَاتِ وَالْأَرْضِ وَمَا بَيْنَهُمَا الْعَزِيزُ الْغَفَّارُ
Source: Ihyaauddeen.co.za