i think it's as much of a coping mechanism as it is a physical need for him, yes. i will admit that my thoughts on the matter are heavily influenced by my own hc of ritsu having chronic fatigue syndrome (like i do) and my personal experience with it, which i feel like are pretty similar to his, but i'll try to remain as neutral as possible.

i do believe that sleeping a lot is inherintly part of his condition, especially considering that as a child and in early stories he has pretty low stamina and canon low blood pressure, it's to be expected that someone with his health issues would need lots of rest. at some point, though, it surely became a form of escapism for him. going by what janus shows us that even as a kid he was always alone in the mansion, he must've started using sleep as a way to make time pass faster, and as time went by and his obvious depression got worse, sleeping probably became an unhealthy coping mechanism. because when he was sleeping he didn't need to hear what his family or the people around him said behind his back, he didn't need to confront the reality where he believed he was being left behind by his brother (and later on by mao too), didn't need to deal with knights infighting or school or even his own health issues. he could just forget everything and live in a blissful dream.

but once you become victim of bad habits like these it's hard to let go. his first year at yumenosaki (the one he had to repeat) is proof of that, he was probably at his lowest back then, and without mao or rei to urge him to get up, go to school or just do his chores, he held onto his need to sleep like a crutch. but you can also see how he slowly stops using that as escapism as the story progresses and knights hold his hands, as he regains his purpose and starts moving forward with his friends. he still sleeps most than the average person, but nowadays i feel like it's more of a necessity due to his condition (like it's supposed to be) than a way of escaping reality, like he has come to terms with it and is now a more balanced thing.

i'm sorry if this is messy, i'm terrible at character analysis and again i was trying not to make my own experiences bleed into this too much heh but i hope it was a good enough answer for you ;u;