Sleep in car on ferry ride

Can you sleep in or even access your car during a ferry ride?

Depends on the ferry. Use the companies website to find the info.

But normally not on most occassions.

Normally not on most occasions, but a lot of ferries in Japan have a section designed so that you can take off your shoes and lie on the floor to take a nap with other passengers.

After some research.

Those overnight long distance ferries seem to have the staff load and unload your car for you from their carparks. So very unlikely that you can access your car in the cargo hold.

The short distance ferries aka Sakurajima allow you to drive on and access you car. But then I am guessing you dont mean this type of ferry.

I only kooked at ferries between Kansai and Kyushu regions. But gonna assume the other half of the country is very similar in practice.