Yes I think he actually sleeps there because throughout the whole poem he talks about how he loves this girl an how beautiful she is. Also by the other Edgar Allen Poe poems and stories I've read by him he seems to talk about death and is very literal about it.
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.
