Wet dreams, also known as nocturnal emissions, are orgasms that happen spontaneously during one's sleep, usually brought on by sex dreams. It's mostly known to happen to men, but women can also have wet dreams, although it's more difficult to know because a female orgasm does not leave ejaculate as evidence.
More About Wet Dreams
Wet dreams are common in teenagers and young adults, but can also happen to older adults, albeit less frequently.
Kinsey reported that 83% of males have experienced wet dreams at least once in their life, compared to 40% of women who report nocturnal orgasms. However, it's more difficult to report female nocturnal orgasms, because the woman needs to be awake (or at least aware) of her orgasm. In 13% of men, a wet dream is their first ejaculation.
Statistics reveal that married or partnered males have fewer wet dreams than un-partnered ones; also, younger males have more frequent wet dreams than older men. However, there is little evidence of a correlation between frequency of masturbation and frequency of wet dreams.
One thing that might influence the frequency of wet dreams is the level of testosterone produced. According to a study by Finkelstein et al, 90% of the subjects taking a high dose of testosterone experienced wet dreams, compared to 13% of the subjects taking none.