This paper explores the themes of darkness, sleep, and the dream world in Heidegger’s work in the late 1920s. In the first part of the paper, I discuss how Heidegger appeals to our pre-understanding of the phenomena of darkness, sleep, and dreams, as well as of light and wakefulness at central points in his analyses. Here as elsewhere, his inquiry is characterised by a hermeneutic circularity: An existentiell pre-understanding of these phenomena informs the existential analysis of Dasein’s being which in turn allow these phenomena themselves to be characterised more closely. In the second part, I discuss how Heidegger acknowledges that the phenomena of sleep and dreams pose a problem for his phenomenology of the world. He notes that sleep brings us before “self-manifesting beings” that “have their own peculiar world-character for the individual, in each case a completely different way in which they world” and even acknowledges that sleep opens up “a broad perspective which has been by no means grasped in its metaphysical intent.” However, he refrains from developing this theme “for fundamental metaphysical reasons” (GA 26 220, MFL 172, GA 29/30, 92, FCM 61, 63). Finally, I broach the themes of darkness and dreams in their most direct and familiar sense with a view exploring their metaphysical possibilities. I first distinguish the experience of the uncanny in darkness from that of anxiety. Against this background, I then draw out the phenomenological implications of Heidegger’s characterisation of the dream world by way of comparison with the kindred phenomena of daydreaming and the mythical world. In conclusion, I discuss why the dream world problematises Heidegger’s understanding of world and of metaphysics in the late 1920s.