You don't need a room or deck space on the overnight ferry. The ferry services offers all night entertainment. I'm not sure if the bars shutdown. I understand that a number of the passengers don't sleep during the overnight sail.
Sample night entertainment offerings on the Silja M/S Romantika:
http://www.tallinksilja.com/documents/10192/21635501/ROM_Spring_ENG-RUS_2016_new.pdf/b2f497aa-dfc0-4e41-9813-853fb2602f71
PUB
When you want to spend your evening in a relaxed atmosphere chatting with your friends, choose the cozy pub which has three different milieus: Library, Irish and Karaoke Pub.
NIGHTCLUB STARLIGHT PALACE
In the Art Deco styled Starlight Palace, you will dance to the best bands and see the fabulous Midnight Show. As the nightclub is in two levels, the views from the second floor to the downstairs dance floor and big stage are fantastic! There is also a Casino in connection to the nightclub, so try your luck in the roulette or slot machines!
9:00 Circus workshop: teachers from Brazil
19:30 Dance Workshop: dance teachers from Brazil
20:00 Party band starts
20:45 Presentation on the city of Tallinn (in Swedish)
21:00 Presentation of tonight’s program and Opening Show from UP LEON
Party band continues
22:00 SHOWTIME! – UP LEON from Brazil - „Summer Feeling“
22:25 CELEBRATIONS*
Party band continues
23:30 SHOWTIME! – UP LEON from Brazil - „Rio by Night“
23:55-01:30 Party band continues
SEASIDE & DANCING
In the Seaside you can dance to live music and quench your thirst with various different kinds of drinks!
18:00 -.... Evegreen music from live artists
21:15 MUSIC GAME with Cruise Manager
23:30 - … KARAOKE
SPACE DISCO
In Space Disco you will dance until morning to the best hit music. During the day, the room is transformed into Sports Bar with big TV screens.
00:00 - 05:00 DJ plays the best hits of all time