A nocturnal emission, informally known as a wet dream, sex dream, nightfall or sleep orgasm, is a spontaneous orgasm during sleep that includes ejaculation for a male, or vaginal wetness or an orgasm (or both) for a female. Nocturnal emissions happen after stressful dreams in REM sleep which activate the sympathetic nervous system hence leading to ejaculation. Nocturnal emissions are most common during adolescence and early young adult years, but they may happen any time after puberty. It is possible for men to wake up during a wet dream or simply to sleep through it, but for women, some researchers have added the requirement that she should also awaken during the orgasm and perceive that the orgasm happened before it counts as a wet dream. Vaginal lubrication alone does not mean that the woman has had an orgasm.