It seems like every time someone goes to sleep in The Lost Hero, they get a vision. There's no mystery about it: Just close your eyes, and talk to your divine parent. It's more reliable than a cell phone or the post office (though, as Leo discovers, sometimes Wheel of Fortune broadcasts may interfere with the signal).
Sleep, then, isn't a window into unreality or vision or spooky mystical stuff. Instead, it's functional—like a car or a socket wrench. And the function is to put the narrative on track, to give the characters and the reader important information, or provide a recap, or just make sure everything works out in the end. Hephaestus appears to Leo in a dream to tell him he can reuse Festus; Aphrodite appears to Piper in order to give her the potion to wipe her dad's memories. Clovis, son of Hypnos, god of sleep, provides the important information that the gods have Greek and Roman aspects, and that a god has stolen Jason's memories. Why would that information be associated with sleep in particular? Just like your day begins and ends with sleep, so do plots points in The Lost Hero.
That's perhaps why the book opens with sleep. "He woke in the backseat of a school bus, not sure where he was, holding hands with a girl he didn't know" (1.2). Coming out of his sleep, Jason enters the narrative. That suggests that sleep is outside the narrative, meaning that when you go to sleep, you're out of the book. From that perspective, sleep is where the author reaches into the world to make some adjustments—to place characters on the right bus, for instance, or make sure they know what's expected of them. Sleep is where you adjust the fantasy so it runs right, which is why dreams, in the book, seem more predictable and less fantastic than reality.