Males have the common experience of awakening with an erect penis, an experience that does not always seem to be tied to the erotic content of their dreams. Studies of this phenomenon, referred to in the relevant literature as nocturnal penile tumescence (NPT), have shown that the incidence of NPT is highly correlated with rapid eye movement (REM) sleep. REM sleep, in turn, is associated with dreaming. Studies have also shown that a similar (but less noticed) phenomenon occurs in the vagina and associated tissue in females. The precise significance of this correlation between tumescence and REM sleep—which, as noted, does not always seem to be directly tied to sexual content in dreams—has yet to be demonstrated.