When it's time to go to sleep, the two seats fold out into a comfy twin-size bed. An upper berth is lowered down from the ceiling to become a second bed above the other, bunk bed style. While the bottom bunk is perfectly comfortable to sleep on, the top is a different story.
After climbing up the steps to get to the top and lie down, a seatbelt-harness-type contraption hanging from the edge of the bed hooks into the ceiling. This is meant to catch you in case you fall off the top bunk while sleeping. Lying on the small top bed can make you a little uneasy, but some people prefer the elevated option.
Another concern I had was how noisy it would be on the train. You get some sounds from going over the tracks at night, but it just turns into white noise. Overall the noise was no bother.