There currently is one option to travel by night train from Warsaw (Poland) to Świnoujście (Poland), taking between approximately 8 and 10 hours: Namely the IC 440. These lines are operated by PKP Intercity and Deutsche Bahn.
PKP Intercity, Poland's national long-distance operator, connects cities across Poland and beyond its borders.
Deutsche Bahn (DB) exited the actual night train business in 2016. However, it operates around a dozen Intercity and Eurocity trains that run exclusively at night.
These trains only have first and second class seats. The cars remain lit throughout the night and the seats cannot be reclined.