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 642-645. Her reign as Saimei encompassed 655-661. 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.
In the history of Japan, Kōgyoku/Saimei was the second of eight women to take on the role of empress regnant. The sole female monarch before Kōgyoku/Saimei was Suiko-tennō. The six women sovereigns reigning after Kōgyoku/Saimei were Jitō, Gemmei, Genshō, Kōken/Shōtoku, Meishō, and Go-Sakuramachi.