There currently are a variety of options to travel by night train from Hämeenlinna (Finland) to Tervola (Finland), taking between approximately 9 and 10 hours: Namely the Yöjuna 265, the Yöjuna 273 and the Yöjuna 267. These lines are operated by VR.
VR, Finland's national railway operator, runs passenger trains throughout the country, including overnight services to northern Finland.