Yeah natural isnt a good word for it. But at the end of the day sleep disorders and ND people exist and have sleep schedules that arent fixed to what capitalist society deems a "natural sleep cycle".
The fact that lots of unemployed people trend towards staying up late and getting up late doesnt seem immaterial to me.
I say "capitalist realism" because it seems like youve internalized the idea of a capitalist work schedule being normal and healthy.
also I should note that during socialist construction in the USSR, the working day was very regimented, it takes a lot of work to build.
I think we are under different material conditions now that would allow for things like flexitime, 4 day work week and shorter work days, and otherwise adapting to different peoples sleep.