In Your Name, O Allah, I Die and I Live
بِاسْمِكَ اللَّهُمَّ أَمُوتُ وَأَحْيَا
Bismika Allahumma amutu wa ahya.
In Your name, O Allah, I die and I live.
What it means
Sleep is described in the Quran as a kind of death, and this dua treats it that way without any drama. You are saying that both states, the one you are entering and the one you hope to wake into, belong to Allah. It is six words, easy to say even when your eyes are already closing.
When and how
Say it once as you lie down, after the other adhkar of sleep. Lying on the right side with the right hand under the cheek is the sunnah posture.
Its virtue
This is what the Prophet ﷺ said when he went to his bed, reported in Sahih al-Bukhari 6324.
Source
Al-Bukhari 6324sahih