There currently are a variety of options to travel by night train from Harviala (Finland) to Simo (Finland), taking between approximately 8 and 10 hours: Namely the Yöjuna 269, the Yöjuna 273, the Yöjuna 265 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.