The word "metaphysical" was applied to a group of seventeenth-century poets by Samuel Johnson. Although the word is seldom used in academic criticism today, it is generally agreed that the metaphysical poets shared certain characteristics: the use of conceits and wordplay, and a sinewy quality in the construction of their verse. Donne's poem "A Nocturnal upon St. Lucy's Day," uses the elaborate conceit of a parallel between the day, the year, and the poet's life, all of which are now at the dead of night. The elaborate comparison is enhanced by diction which emphasizes the shrunken, shriveled state of the natural world in midwinter, a condition which reflects that of the poet's heart. These attributes make for an archetypal metaphysical love poem, a genre which often includes multiple references to death (think, for instance, of Marvell's "To His Coy Mistress," where the addressee is still alive).
The tone of Donne's is more classically elegiac than English elegy normally is, being bitter, like a Greek elegy, rather than mellow and melancholy. However, the tone softens at the end to conform to the conventions of English elegy, as the poet expresses good wishes to those who will still be able to enjoy their love when summer comes. As for him, he is a shadow or a husk of humanity, no longer a man. The absence of his beloved used to make them both "carcasses" when she was still alive, and this experience foreshadowed his permanent state now, as well as hers.