Empress Kōgyoku (皇極天皇 Kōgyoku-tennō), also known as Empress Saimei (斉明天皇 Saimei-tennō), was the 35th and 37th monarch of Japan, according to the traditional order of succession.
Kōgyoku's reign spanned the years from 642 to 645. Her reign as Saimei encompassed 655 to 661. In other words:
642: She ascended the throne as Kōgyoku-tennō, and she stepped down in response to the assassination of Soga no Iruka.
645: She abdicated in favor of her brother, who would become known as Emperor Kōtoku.
654: Kōtoku died; and the throne was vacant.
655: She re-ascended, beginning a new reign as Saimei-tennō.
661: Saimei ruled until her death caused the throne to be vacant again.
The two reigns of this one woman spanned the years from 642 through 661.