Since していました is used to indicate that an action took place during a specific time, it is often accompanied by a time or else another action or event that occurred at a certain time.
Kinou, benkyō shimashita. Kinou no ni-ji goro, benkyō shite imashita.
I studied yesterday. Around two o’clock yesterday, I was studying.
Also note that ～ている cannot be used with momentary actions because it implies a continuous action.
Jyugyō-chuu ni, nete imashita. Jyugyō-chuu ni keshigomu o otoshimashita.
I was sleeping during class. I dropped my eraser during class.