Is it a condition for praying in the latter part of the night to sleep after 'Isha', or is it permissible to wait for the prayer without sleeping?

Voluntary prayer at the end of the night is a great act of worship. Sleeping before it is not a condition to attain the reward of night vigil (qiyam al-layl). However, it is better for the vigil to be after sleep; this is called tahajjud, and it is the best type of voluntary prayer absolutely. As for one who performs vigil at the beginning of the night before sleeping, he has not performed nashiat al-layl. If someone intentionally wakes up and performs night vigil without having slept, he will receive the reward of night vigil and the reward for his intention and eagerness for worship.