Is ablution for the 'Isha prayer sufficient for sleeping in a state of purity, or is a special ablution required before sleeping? And is it necessary to renew ablution if it is invalidated before sleeping for one who wishes to revive the Sunnah of sleeping in a state of purity?

Ablution before sleep is a confirmed Sunnah of the Prophet, peace and blessings be upon him, and its purpose is to sleep in a state of purity. If a person is already in a state of ablution, for example, from the ablution for the 'Isha prayer, and remains in that state of purity, then that suffices him and he has observed the Sunnah. If he gets up to relieve himself after lying down in a state of ablution, it is recommended for him to perform ablution again so as to sleep in a state of purity. However, if his ablution breaks while he is in bed after having lain down in a state of ablution, then his initial ablution suffices him, and he will have observed the Sunnah.