At the very beginning of the book, we're told about Serena's dreams: "Serena's dreams had been especially troubling last night" (1.3). Um, okay. It seems like a weird time to stop and talk Freud to us.
Dreams are a big deal in the book because they foreshadow what's to come. For instance, Pemberton dreams about his baby dying long before Serena threatens Jacob's life. And did you notice that Serena has nightmares about her family dying in Colorado before something big happens, like right before Pemberton kills Harmon or when she first comes to the camp to live with Pemberton? Her dreams remind her of her troubled past and hint at the dark future.
Before long, though, Serena's dreams stop. Check out what we're told about them:
She slept well now, in a deepness beyond dreams, she claimed. It had been that way since she'd stayed in the stable with the eagle, as though the nightmares had come those two sleepless nights and, with no dream to enter, gone elsewhere, the way ghosts might who find a house they've haunted suddenly vacated. (12.17)
Training the eagle gives Serena more power, so it's no coincidence that her dreams go away once she feels more in control. Since she has nightmares when she's vulnerable, we can see that she no longer feels susceptible to the things around her at this point in the book. This is when we notice her gaining more influence as a business partner, too. No more dreams for this lady—her reality gives her what she needs.