“Whoever
among you fears that he will not wake up at the end of the
night, let
him pray Witr at the beginning of the night, then go to
sleep.
Whoever hopes that he will wake up at the end of the night, let
him
pray Witr at the end of the night, for recitation (of the Qur’an)
at the end of the night is attended (by the angels), and that is
better.”
Sunan Ibn Majah Book 5, Hadith 385