Are Surat Al-Mulk and Al-Sajdah recited at any time of night, or only upon going to bed?

The Prophet, peace and blessings be upon him, used to recite Surah As-Sajdah and Surah Al-Mulk before sleeping. This means he would not sleep until after reciting them, whether that was immediately before sleeping or some time before it, because the intent is not to sleep before reciting them. Their recitation is recommended, not obligatory.