I say, with Allah's guidance: It is recommended to delay the Witr prayer to the end of the Isha time for those who are confident they will wake up, so that it can be the conclusion of the night prayer. As narrated by Jabir, may Allah be pleased with him, the Prophet (peace be upon him) said: 'Whoever fears that they will not wake up at the end of the night should pray Witr at its beginning, and whoever hopes to wake up at its end should pray Witr at the end of the night, for the prayer at the end of the night is witnessed, and that is better.' (Sahih Muslim 1: 520). Refer to: Al-Wiqaya p.137, and Tabyin Al-Haqa'iq 1: 84, and Allah knows best.