The primary conceit in "A Nocturnal upon St. Lucy's Day" is the comparison the speaker makes between the setting, which is the shortest day of the year, and the loss of his beloved. As the poem opens, the speaker notes that the sun is so exhausted that it can no longer be depended on to provide a steady and constant light—which, of course, is its sole purpose. On this dark and cold December day, the entire earth has "sunk" away, as all forms of life have buried themselves underneath the cold earth.
In the fourth stanza, the comparison is fully realized as the speaker reflects that he has been transformed into a "nothing" by "her death" (italics added for emphasis). He believes that even the dying December plants are more alive than he is after his profound loss and that now he is stuck in his own eternal winter, a place from which the metaphorical sun of experiencing love will never shine on him again. He has lost his sense of purpose, much the same as the sun has forgotten that it exists to give light and warm the earth. The speaker, like the sun, is too weary to continue and sputters in the remnants of his existence.
Imagery heightens the cold and desolate tone. The earth, which was once alive at the height of summer, is now "interr'd," conjuring images of tombs. The speaker recalls that while his beloved was alive, being separated turned them into "carcasses"; their souls died when not together. Again, this presents a chilling image of death and foreshadows the speaker's own forlorn future. Images of darkness, death, and decay are woven throughout the poem to emphasize the speaker's feeling that without his beloved, he is now "every dead thing."