Is performing ablution before sleep an obligation or a Sunnah, what is its merit, and what is light ablution and how is it performed?

Ablution before sleep is a Sunnah, not an obligation. Among its virtues is that whoever sleeps in a state of purity will have an angel with him seeking forgiveness for him, as stated in the Hadith: "Whoever sleeps in a state of purity, an angel stays with him in his garment. He does not wake up but the angel says: 'O Allah, forgive your servant so-and-so, for he slept in a state of purity.'" Light ablution is when the person performing ablution washes each limb once while completing the ablution properly. It does not differ from a complete ablution in method or order, but it is limited to the sufficient amount.