In first class there are two seats on one side of the aisle and one seat on the other side. In second class there are two seats on either side of the aisle. As the name says, these trains are for overnight travel and you should consider reserving sleeping accommodations such as a sleeper or a couchette. In the morning you'll wake up refreshed and ready to go. Sleepers contain berths, with a private washstand, fresh linens and towels.
First class accommodates one or two people.
Second class accommodates from two to four people. Couples, families and travelers of the same sex can share a sleeper. Couchettes are open bunks in a compartment each with a pillow and blanket. Usually located in second class, they accommodate up to six people. There is no distinction by sex, so you should expect to sleep in your daytime clothes.