The theme in "Ode to a Nightmare" by John Keats is the speaker's urge to leave the real world. In the beginning of the poem, the speaker explains that his mental state is intoxicated and that his heart aches, identifying that the theme of the poem will not be cheerful.
The poem explains from the speaker's standpoint that his problem with the real world is that nothing lasts forever. The third stanza of the poem indicates that he might be referring to beauty and love not lasting forever. In "Ode to a Nightmare," Keats focuses on the feelings of numbness, life, death, immortality, the actual and the ideal and the nature of human life.