The Prophet, peace and blessings of Allaah be upon him, said:
“Do not pray behind one who is sleeping or one who is talking.” (Reported by Abu Dawood, no. 694)
This is because one who is talking will distract the worshipper with their talk, and one who is sleeping may expose something that will distract the worshipper.
Al-Khattaabi, may Allaah have mercy on him, said: “As for praying behind people who are talking, al-Shaafa’i and Ahmad ibn Hanbal considered this to be makrooh or disliked, because their talk distracts the worshipper from his prayer.” (‘Awn al-Ma’bood, 2/388).
As regards not praying behind someone who is sleeping, a number of scholars thought that the evidence for this was weak (including Abu Dawood in his Sunan, Kitaab al-Salaah, Tafree’ Abwaab al-Witr, Baab al-Du’aa’, and Ibn Hajar in Fath al-Baari, Sharh Baab al-Salaah khalf al-Naa’im, Kitaab al-Salaah).
Al-Bukhaari, may Allaah have mercy on him, quoted the hadeeth of A’ishah in his Saheeh, Baab al-Salaah khalf al-Naa’im: “The Prophet (peace and blessings of Allaah be upon him) used to pray whilst I was lying across from him on his bed…” (Saheeh al-Bukhaari, Kitaab al-Salaah).
Mujaahid, Tawus and Malik thought it makrooh to pray facing someone who was sleeping, because they may expose something that would distract the worshipper from their prayer. (Fath al-Baari, ibid.)
If there is no risk of that happening, then it is not makrooh to pray behind someone who is sleeping. And Allah knows best.