In addition, women who have orgasms during sleep usually have them several times a year. Dr. Kinsey and his colleagues defined female nocturnal orgasm as sexual arousal during sleep that awakens one to perceive the experience of orgasm. Girls and women who don't have orgasms in their sleep, or who don't know whether or not they've had them, are perfectly normal. Vaginal secretions could be a sign of sexual arousal without orgasm.
Similar studies find that a much higher percentage of boys and men experience wet dreams.
This, combined with a greater focus on male sexuality by science and the public in general, are probably two big reasons why we don't hear very much about women's nighttime orgasms. In fact, Alice thinks this is the first time this question has been posed here. Thanks for asking.