The Ruling on performing Witr before Sleep
Question :
I am a woman, who, when I lie down to sleep, becomes very tired. Is it permissible for me to offer Witr before sleeping, because I do not wake until it is time for the Fajr prayer? And will this be credited as the night prayer for me?
Answer:
If it is your habit not to wake until the Athan of Fajr, then it is better to advance the prayer which you wish to perform to the time before you sleep, because the Prophet advised Abu Hurairah to offer Witr before sleeping.
So perform the prayers which Allah has made incumbent upon you and then offer Witr before sleeping and sleep upon Witr. And if you are able to wake before the Athan of Fajr, and you wish to offer supererogatory prayers, there is no sin for you to pray in units of two Rak'ahs, but without repeating Witr.
Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 2 Page 318