Within the depth-psychology corpus, ‘Day and Night’ operates simultaneously as a cosmological given, a psychological archetype, and an alchemical cipher. Jung establishes the foundational claim: the daily alternation of light and darkness has impressed itself upon the psyche from primordial times, generating mythic images of the solar hero’s diurnal journey rather than mere meteorological facts. Campbell elaborates this by linking the nocturnal realm to dream-logic—a world where objects shine of themselves and transformation proceeds by non-mechanical means—positioning Night as the matrix of mythic saturation. Hillman, characteristically, rescues Night from devaluation, insisting that Nyx and her offspring demand discriminating engagement rather than pharmaceutical suppression, and reading the elder’s disrupted sleep as initiatory exposure to the underworld’s pedagogy. The alchemical tradition, represented by von Franz’s commentary on the Aurora Consurgens, locates dawn—the precise threshold between day and night—as the privileged moment of the opus, carrying both temporal and chromatic significance. Plato’s cosmological discussions in the Timaeus assign Earth the role of ‘guardian and maker of day and night,’ grounding the alternation in rational celestial mechanics. Neumann reads the polarity through the Great Mother, in whose matriarchal sphere the sun is born and dies within the arc of light that always ends in nocturnal darkness. Corbin’s Sufi material inverts the polarity altogether, proposing a ‘luminous Night’ and a ‘dark Midday’ that displace conventional categories. The term thus anchors discussions of consciousness and unconscious, solar mythology, alchemical process, and mystical epistemology.