In The Giver by Lois Lowry, how does Jonas's answer to his mother's question about his sleep and dreams reveal a change in him?

We know from Chapter 5 that Jonas always reports his dreams faithfully and fully to his family, just like he's supposed to. Jonas's actions show he obeys the rules and fully opens up his mind and his feelings to his family. The result is that his community's rules continue to control him: after revealing his dream in which he felt his first "stirrings" of romantic and sexual desire, Jonas is prescribed a lifelong regimen of pills that kills those natural desires, just like every other adolescent and adult in the community takes.

Then, in Chapter 9, Jonas is "prohibited from dream-telling," as part of his new job as the Receiver. This means his new job will cause him to dream about secret or forbidden things, and, unprecedentedly, he won't be required to reveal them to his family, and, even more amazingly, Jonas is now allowed to lie.

So we're not too surprised when, in Chapter 12, Jonas's mom asks him if he slept well and if he had any dreams. Jonas answers by saying "I slept very soundly." It's true Jonas slept well, but he did have dreams — and so by not telling his mother about them, he was basically lying to her.

What Jonas's response to his mother reveals is that Jonas is becoming more willing to abandon the rules he's always followed. He's more willing to accept the liberating rules that guide his role as the Receiver. He's willing to hide his dreams, and he's almost willing to tell lies. Most interestingly, the dream Jonas hides from his mother was about longing for a place beyond the community. We understand, then, that Jonas is realizing there may be a world worth knowing that lies outside of his limited, highly-regulated life within his community.