How does Daniel know this? Oh, yeah—he remembers it from one (or several) of Luce's past lives. Does Luce pick up on this? Nah. She wouldn't be a YA heroine if she did.

From this interaction, it's clear that this is a tipping point for Daniel. Every time Luce says this to him, it must put his guard up, because it's a signal that she might disappear at any moment. That really puts a damper on an otherwise uneventful evening, doesn't it?

Here we see Daniel (not yet named) showing how much he cares for Luce (also not yet named, since we're in Prologue Land)—and how much he remembers about her character. Learning and remembering someone's eating habits is an intimate thing, and Daniel's concern for Luce's sleeplessness, paired with his wish to help her fall asleep, is especially telling: it suggests that he knows a great deal about her and cares for her a lot. It's also the only instance in the story when Daniel and food are connected; we usually Cam and food connected instead. Basically, Cam + food = potential danger, while Daniel + food = comfort and safety.