He who amongst you is afraid that he may not be able to get up at the end of the night should observe Witr (in the first part) and then sleep, and he who is confident of getting up and praying at night (i.e. Tahajjud prayer) should observe it at the end of it, for the recitation at the end of the night is witnessed*, and that is better.

*: meaning, "by angels"

(Sharh an-Nawawi)