There currently are a variety of options to travel by night train from Rovaniemi (Finland) to Riihimäki (Finland), taking between approximately 10 and 14 hours: Namely the Yöjuna 264, the Yöjuna 266 and the Yöjuna 274. These lines are operated by VR.
VR, Finland's national railway operator, runs passenger trains throughout the country, including overnight services to northern Finland.