A tired reader sits with a book, trying to stay awake, and the poem catches the strange mental fog that comes just before sleep. The title, "Soporific," means something that makes you drowsy, and the speaker seems to be both the one reading and the thing being put under.
The images are plain and a little unsettling. "White bread for dinner" suggests something bland and nourishing in no way at all, and "the house of sleep" turns drowsiness into a place the speaker has wandered into. Then the mind "gropes" for its way through the book, as if the words have become a dark hallway. The last lines shift inward: a "great raw nothing" moves inside the speaker. It's hard to say if that feels peaceful or frightening, and that uncertainty is the point. The mood is hushed, dim, and faintly uneasy, like losing your grip on thought while still half aware of it.