I've found several sources that explain that mangakas (manga artists) have to work insanely long hours to meet their deadlines.
In this Quora answer, an assistant mangaka claims:
We were well-paid, but worked 16-hour days , about 20-25 days per month.
On the same page, another answerer says
The life of mangaka is basically non-stop working: about 14-15 hours every day in drawing, very popular mangakas sleeping about 3-4 hours of a day and have several hours of a week free time.
This blog article quotes the schedules for several mangaka of highly successful manga series.
Oda only sleeps 3 hours a day.
Here's the time schedule of Kishimoto when he was writing Naruto. On his inking days, the time spent on his desk totalled 19 hours a day.
From personal experience of working over 55 hours in a week, I find the claim to be unlikely.
Is this the the usual amount of work mangakas have to do in order to meet their deadlines?