There currently are a variety of options to travel by night train from Helsinki (Finland) to Rovaniemi (Finland), taking between approximately 11 and 13 hours: Namely the Yöjuna 265, the Yöjuna 273 and the Yöjuna 267. These lines are operated by VR.
VR is Finland's national railway company, operating passenger trains across Finland, including night trains connecting the south of the country with the north.