Shaikh Bin Baz rahimahullah was asked:
Are the remembrances for sleep specifically for sleep at night only? If a person gets up to relieve himself or to drink water, does he repeat what he said of remembrances?
What is apparent is it suffices if he says it at the beginning when he sleeps. It suffices, (and) there is no harm. However, the Sunnah is achieved by the remembrances which he says and the supplication he makes when sleeping at the beginning of sleep. What is specifically for the night and the Prophet sallahu alyhi wa salam clarified that it be said when he wants to sleep, then this is specified for the night. And what has not been reported with specification, then this is general for every time. As for what has come specifically for when he wants to sleep at night, then this is his Sunnah at night if he wants to sleep at night”.
[Noor ala Darb no. 775]
Faisal Ibn Abdul Qaadir Ibn Hassan