Julio Cortazar creates a blur between what is real and what is not in "The Night Face Up". While he tells the story of a man who shifts from one world to the other, he gives the readers reasons that make it hard to distinguish the dream world from the real one. In my defense, I suggest that the real world in the story is the modern world in which the protagonist had an accident and was taken to the hospital, and the dream world is the Aztec world in which the protagonist is a Moteca who runs away from the Aztecs during the War of Blossom.
To begin with, the fact that the protagonist snoozes every time he shifts from the modern world to the Aztec world is what usually happens when one starts to dream. When people start to sleep, they slowly lose their sense of self and become less aware of their surroundings. This happens to the protagonist in the story just before drifting into the Aztec world. For example, after eating his meal in the hospital, the protagonist allows himself to sleep, and then he finds himself running in the Aztec world (71), or while thinking about the accident, he starts to get sleepy, and the moment of transition is described like this: "The violet light of the lamp up there was beginning to get dimmer and dimmer" (73). The opposite of this occurs when he passes from the Aztec world to the modern world. He turns back suddenly, just as it happens when he wakes up in the hospital bed "in a single jump" after seeing himself carried by priests to be sacrificed (75). This is something like waking up from a nightmare with a sharp move.
Secondly, his moves in the Aztec world affect his moves in the modern world, which often happens while having a nightmare. To put it another way, if someone is having a realistic dream and sees themselves running, they will probably start moving their legs, or if they see themselves falling, they will probably jump in bed to get rid of that feeling. Just like this, the protagonist wakes up from his dream in which he "leaped forward desperately" (70) to run away from the Aztecs. Seeing this, the patient beside him warns him, saying, "You‘re going to fall off the bed" and "Stop bouncing around, old buddy" (70). Something resembling this happens towards the end of the story, as well, while he is being carried by the priests to his death. The priests hold him with his face up and "his head hanging down backward" (75) while carrying him to the place where they will sacrifice him. As a result of this, he wakes up in his hospital bed motionless except for his head, which is dangling (76).
Also, the events he experiences in the modern world reverberate in the Aztec world. It is a fact that the real world has its effects on the dream world. An incident that happens to someone might occur in their dream or it might occur by alternating itself in that person’s mind. This is what the protagonist lives through in the story. He lies on his back in both worlds, being carried facing the sky, being cut with a lancet in the surgery and with a knife in the temple, and many other things happen in the Aztec world after they happen in the modern world. For example, he is first described as lying on his back on his hospital bed with these words: "Still on his back, so a little uncomfortable [...]" (71). Later, when he wakes up in the dungeon of the Aztecs, he finds himself "sleeping on his back" (73). To extend the example, he is also carried to the hospital while facing the sky, "rolling along under trees full of birds" (68), and in parallel, he is carried in the Aztec world the same way it is indicated in these words: "Face up, under a mile of living rock which, for a succession of moments, was lit up by a glimmer of torchlight" (74).
To summarize, I believe that the Aztec world is the story's dream world, which occurs as a result of the effects of the modern world's accidents; when the protagonist has a bad feeling from things like lying uncomfortably on the bed, it reflects the dream world, and when he hstruggledSonrales in the dream world, he is affected by them in the real world. Some may also suggest that the Aztec world is the reality by referring to the end of the story because the narrator accepts what comes last as the real unlike the reader (me) who perceives what comes first as the real. But, considering the narrator's half-conscious and wounded state, I can indicate that his perception was damaged, which makes his statement about reality unreliable.
P.S. In the short movie adaptation of the story, it is implied that the real world was the Aztec one, but, then again, the movie is animated based on the narrator's point of view. Thus, the conclusion is unreliable. Regardless, it is a good adaptation and reflects the story very well.