What is happening to the narrator in "Premature Burial"?

Edgar Allan Poe's "Premature Burial" first tells sundry tales of people being buried alive. These individuals include a one Mademoiselle Victorine Lafourcade, an unhappily married woman, who was dug up by her former lover after she was supposed to be dead. This woman revitalized at her lover's touch. Another man (an artillery officer) was thrown from a horse and pronounced dead. He nevertheless managed to break out of his coffin and alert a visitor to the cemetery by means of shaking the earth. Additionally, one young attorney, Mr. Edward Stapleton, was pronounced dead from typhus fever. He nevertheless came back to life despite having undergone a partial dissection, during which he started to speak.
After having told the stories of these several individuals, the narrator begins his own tale. He explains that he suffers from catalepsy, a condition wherein one's limbs remain involuntarily locked in place during period seizures. The narrator explains that he occasionally wakes up in the evening and is unable to move his body for several minutes. Reflecting a common fear during the era in which Poe was writing, the narrator explains that he was preoccupied with being buried alive.
One night, he dreams that a grim figure visits him and shows him a group of graves thrown open, so that the narrator can see the decayed bodies. The narrator is struck with such terror that he makes his close friends promise not to bury him until his body shows signs of decay. He also designs his coffin with a bell that may be rung from the inside.
Nevertheless, despite his precautions, the narrator awakens after a seizure to find that he has been buried in a different vault. He sees nothing but darkness, feels his jaw shut with a cloth (as is usually performed on the dead), and is surrounded by wood. He attracts the attention of several men who remind me that he has just been on a hunting trip and slept in the cabin of a wooden ship. The cloth around his jaw is not a bandage but a nightcap. From the episode, the narrator realizes the futility of living in fear.
In conclusion, the narrator was not in fact buried alive, but he fell asleep in the cabin of a ship.