For the naturalistic writer, .

a. scientific laws govern individual instances
b. individual instances are suggestions or symbols of higher truth
c. what happens is the operation of a higher truth
d. symbols make individual instances real

There is evidence for both views (that he sleeps there, or that it's his imagination) and I'll write them here.
a. He actually sleeps in his love's tomb because Annabel Lee was the only person he cared about, and her loss actually drove him crazy, so he has to spend every night with her. He can't bear being separated from Annabel Lee, "my darling - my darling - my life and my bride," so he lies down by her side every night. Also, they were children when they first fell in love, so after Annabel's death, his mind might have stayed a child's mind, and become so demented that he has to sleep in her sepulcher. In his demented mind, he believes that Annabel is still alive, and that he goes to sleep with her every night.

b. He doesn't sleep in Annabel Lee's sepulcher; it's cold and dark in there! This is from his imagination because the lines before it are all figurative (dreaming of Annabel, and seeing her bright eyes in the stars). The symbolic meaning of lying down by her tomb means that he wants to always stay by her side, and die when she dies (when he lies down in a tomb, it symbolizes him dying). This powerful metaphor emphasizes the poet's undying devotion and love toward Annabel Lee.