Well, in fanfictions, Naruto might always be sleeping because it could be a plot device. For example, it could lead to him having prophetic dreams. Also, it could be a way to make other characters worry about him. His friends might think he's ill or something's wrong with his chakra. And perhaps the author just wants to show a different, more vulnerable side of Naruto that we don't often see in the original series.
One reason could be that he's overworked from all his ninja training and missions. In fanfictions, authors might use this to show the toll it takes on him. Another could be that he's having some sort of internal struggle, like nightmares related to his past traumas, and sleeping is his way of escaping or dealing with it. Maybe he's also been affected by some sort of jutsu that makes him sleepy all the time.
In some fanfictions, Naruto's constant sleeping could be due to a curse. This curse might be placed on him by an enemy he made during his travels. It's also possible that he's simply exhausted from constantly trying to improve himself and gain the respect of the village. The fanfiction author might use this as a way to explore how the other characters react to Naruto being in such a state. They could either take advantage of it or try to help him, which can lead to different story arcs.