For me, it tends to be characters, but it could be plot. Most often, the books that keep me up at night are character driven, with a side note of suspense plot. I like them, I identify with them, and I want to know what happens to them (more often than not, whether or not something imminent and bad happens to them). Plots can occasionally do that; I read Stroud's Gates of Ptolemy, HP7 and Atonement quickly for plot rather than character. But usually it's a character that I find compelling (The Hunger Games and Fire, for example). Alternately, I've read lots of books with compelling characters, but I don't often stay up to find out what happens to them if they aren't in danger of something bad happening.

Conversely, craft I love, but it doesn't keep me up at night.