If I know that I will be going to sleep that night and that it will be very late (say, 4-5 am), I usually say HaMapil around 1-2 am to guarantee that I don’t accidentally fall asleep without it.
Someone who is up all night is a Machlokes HaPoskim. I think the Tzitz Eliezer or R’ Ovadia has a Tshuvah where he brings down the relevant sources.