Abigail's "Hearts" is about missing someone and being haunted by what was left unsaid between you. The speaker says their heart barely sleeps, and when it does, it fills with nightmares about "you and the things we said." The line "nightmares are dreams that forgot to fall asleep" is the poem's sharpest image: a bad dream is just a thought that never got to rest.
The structure helps carry this feeling. Each stanza picks up the last word of the one before it (dreams, haunting, fall asleep, grief), so the thoughts loop and chain together, much like a mind that can't switch off. The speaker also argues that things "left alone or told" are better than "thoughts unknown," suggesting that uncertainty hurts most.
By the end, the mood is quiet fear and regret. The speaker finds loving this person easy, but being without them frightening. Saying goodbye, the final lines admit, was easier than actually living with it.