You may want to make love all night long, but your body most likely doesn't, studies show.
Studies at two German universities found that the hormone prolactin may dampen sexual arousal after orgasm, perhaps signaling to the body that it's had enough. Researchers, led by Michael Exton, Ph.D., a biological psychologist at the University of Essen's Institute of Medical Psychology, asked 10 women to masturbate until achieving orgasm, then examined them afterward. He discovered a surge in the hormones adrenaline, nonadrenaline and prolactin that occurred during arousal and orgasm--but prolactin's rise was the most dramatic and prolonged.
Prolactin has been linked to functions in both men and women, including sperm and breast milk production. Exton believes it regulates dopamine, a neurotransmitter that plays a role in movement control, pleasure and pain, and likens it to a built-in switch for turning on and off sexual desire. "The prolactin surge may possibly signal the brain and reproductive organs that 'once is enough,'" he says.