There currently are a variety of options to travel by night train from Rovaniemi (Finland) to Hämeenlinna (Finland), taking between approximately 9 and 14 hours: Namely the Yöjuna 264, the Yöjuna 266 and the Yöjuna 274. 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.