How early can you board the hamanasu train

In the US, overnight trains allow you to board and get settled in your sleep area at least an hour before departure time. Does that happen in Japan, too. If I am taking the 22:00 Hamanasu out of Sapporo, what is the earliest i can expect to be able to get on?

The train is scheduled to enter platform #5 of Sapporo Station at 21:38. I believe that boarding will become possible right away. If you travel by unreserved seat, you can line up on the platform long before 21:38 if competition is high. Signs on the platform indicate where the doors of each car will be located. People will line up behind these signs.

From my experience, I remember that I could board the old train soon after the trains arrived. I recommend buying bento boxes and hot drinks before you board.