I'd also like to add a tiny bit of history to this.
One of the major reasons why mangakas have shit hours is because of Osamu Tezuka.
The father of manga worked himself to the bone, drawing not only Astro Boy (Tetsuwan no Atom), but also putting out other works. His workload and work "ethic" is basically the reason why being a mangaka is worse than working at McDonalds. And Japanese culture, in general, made the issue worse without realizing that Tezuka was... special.