One possible reason could be the stress of his ninja duties. He has so many missions and responsibilities that his mind is constantly racing, making it hard to relax and fall asleep.
One possible reason could be that he was severely injured in a battle. Maybe he got his legs broken or had some internal damage that affected his ability to move them. For example, if he fought against a really powerful enemy who used a jutsu specifically targeting his legs, it could leave him unable to walk.
Maybe he had a bad experience as a kid. Like he almost drowned once, and that made him too scared to learn how to swim properly.
Stress and anxiety can often keep people awake. Many have shared stories of how work pressure or personal problems caused them to toss and turn at night.
One possible reason could be the loss of his friends. Naruto values his friends deeply, and if he were to lose someone like Sasuke again, it would be devastating. Another reason might be the pressure of being the Hokage. The responsibilities are huge, and he might feel like he's not living up to the expectations of the village. Also, memories of his lonely childhood could resurface, making him feel sad and alone, leading to him crying himself to sleep.
One possible reason could be a powerful jutsu gone wrong. Maybe he was hit by an enemy's attack that was specifically designed to numb his senses. Another reason might be some kind of psychological trauma. For example, if he witnessed something extremely horrible, his mind might have shut down his ability to feel as a form of self - protection.
One possible reason could be his deep concern for Naruto. Despite their complex relationship, Sasuke might be worried about Naruto's well - being after all they've been through. He could be silently guarding him while he sleeps, a sign of his unspoken affection and protectiveness.
Maybe in the fanfiction, baby Naruto has a nightmare. Kushina then holds him close, reassuring him that he's safe. She could tell him stories about his father, Minato, and how they both love him very much. This would comfort Naruto and he'd soon fall back asleep in her warm embrace.
Maybe he's worried about her. Clarke is often in dangerous situations, and Bellamy might be keeping an eye on her to make sure she's okay while she rests.
The fanfiction might use his sleeplessness to explore his inner thoughts and feelings. Maybe he reflects on his journey, his growth, and the people he has lost or gained. This self - reflection during his sleepless nights can give readers a more in - depth understanding of Naruto as a person, not just as a ninja.
In some fanfictions, it could be related to the magical core inside Harry. Maybe his magical core was damaged in some way, and over time, it weakened to the point where his body couldn't sustain itself anymore. Since sleep is a vulnerable state, that's when his body finally gives in. Also, it could be part of a complex plot where the author wants to explore the reactions of other characters to Harry's sudden and unexpected death while sleeping. It can create a lot of drama and angst among the remaining characters in the story.