Sunan Abi Dawud Book 5, Hadith 57
The Messenger of Allah (ﷺ) as saying: When one you sleeps, the devil ties three knots at the back of his neck, sealing every knot with, "You have a long night, so sleep." So if one awakes and mentions Allah, a knot will be loosened; if he performs ablution another knot will be loosened; and if he prays, the third knot will be loosened; and in the morning he will be active and in good spirits; otherwise he will be in bad spirits and sluggish.
When the opening verses of Surah Al-muzammil was revealed, the Companions would pray as long as they would pray during Ramadan until its last verses were revealed. The period between the revelation of its opening and the last verses was one year.
Do not give up prayer at night, for the Messenger of Allah (ﷺ) would not leave it. Whenever he fell ill or lethargic, he would offer it sitting.