A nocturne is a poem set at night.
From A Poet’s Glossary
The following definition of the term nocturne is reprinted from A Poet's Glossary by Edward Hirsch.
A night scene. John Donne was the first English poet to employ the term nocturnal to designate a genre in “A Nocturnal upon S. Lucy’s Day, being the shortest day” (1633). Donne sets his poem at midnight (“’Tis the year’s midnight, and it is the day’s”) and creates an elegy on the shortest day of the year, the winter solstice, by borrowing from the night offices of the Roman Catholic canonical hours. In early church writings, the term nocturnes (Nocturni or Nocturna) refers to “night prayer” or night vigil. The notion of associating night with spiritual contemplation goes back at least as far as the Neo-Platonists. “I shall sing of Night, mother of gods and men,” one Orphic hymn begins. “The night is often the secret site of initiation, purification, and other threshold activities bridging the relation between what is human and what is not human and providing a context for changed roles and states of being,” Susan Stewart writes, pointing to the Japanese tradition of night poems as well as to the Navajo tradition of yerbichai, or “night chants,” sung during Night Way rituals. The nocturne became a European musical type in the nineteenth century, a pensive, moody instrumental piece especially suitable for playing at night, and thereafter poetic nocturnes frequently evoke the melancholy feelings or tonalities of piano nocturnes.
One could make a good international anthology of the modern poetic nocturne, which is frequently a threshold poem that puts us in the presence of nothingness or God—it returns us to origins—and stirs poets toward song. It often flows from an urban sensibility. Charles Baudelaire considered calling his book of prose poems Poèmes nocturnes (Nocturnal poems). Nocturnes are often poems of sleeplessness, the cry of the solitary and bereft ensouled in poetic form (Rubén Dario’s “Nocturne,” which begins “You who have sounded the heart of the night,” 1905; Federico García Lorca’s “Sleepless Night [Brooklyn Bridge Nocturne],” 1929; Marina Tsvetaeva’s “Insomnia,” 1923). Many are elegies, as in Gabriela Mistral’s Tala (1938), which includes a series of mystical “nocturnos,” graveside meditations occasioned by her mother’s death, and Léopold Sédar Senghor’s book of intimacies, Nocturnes (1961).
Midnight is often the witching hour. At this culminating moment in the nocturnal realm, everything must be let go that is associated with day or daylight mind. Rather, the mind is now loosened for reverie and illumination. Walt Whitman’s “A Clear Midnight” (1881) is an incantation that delivers a sense of overpowering spiritual immensity:
Thus is thy hour O Soul, thy free flight into the wordless,
Away from books, away from art, the day erased, the
Thee fully forth emerging, silent, gazing, pondering
the themes thou lovest best,
Night, sleep, death and the stars.