New research provides evidence that sexual intercourse has greater sedative properties for women than it does for men. The findings have been published in Evolutionary Behavioral Sciences.

The study of 128 female and 98 male college students found that women reported a higher likelihood of falling asleep after heterosexual penile–vaginal intercourse than did men. Women were also more likely to report falling asleep before their partner compared to men. Being inseminated was positively associated falling asleep after orgasm as well.

“Our paper summarizes growing evidence that the common designation for having sex, ‘getting laid’, maps on to an evolved adaptive mechanism that functions to promote sperm retention in humans, which as a species are unique in having evolved an upright posture and bipedalism. This brings the woman’s reproductive tract into a perpendicular orientation with respect to gravity where it may not be suitable for the retention of sperm,” study author Gordon G. Gallup, Jr., a professor at State University of New York at Albany, told PsyPost.

“Having sex while laying down with the man on top brings the female reproductive tract back into a more primate parallel orientation with gravity, and women that remain in a spline position after sex would therefore have a greater chance of getting pregnant. Consistent with this analysis, we found that women were unwittingly more likely to fall asleep after sex than men.”

The findings run counter to a previous study on the topic, which found no differences in perceived sleep quality or latency between men and women when sex with a partner involved an orgasm. Another study found men and women fell asleep at roughly the same time after sex, but women were more likely to fall asleep first when sex hadn’t taken place.

However, while the previous research examined sexual activity in general, the current study focused specifically on penile–vaginal intercourse that occurred at night. “Because of the demands of work, school, and a variety of other activities during the day, it is less likely that people will succumb to the temptation to fall asleep after daytime sex, and by failing to distinguish between the effects of sex during the day and the effects at night, other studies may have unwittingly masked the existence of otherwise subtle but important sex differences,” Gallup and his colleagues explained in the study.

The study, “Sex Differences in the Sedative Properties of Heterosexual Intercourse“, was authored by Gordon G. Gallup, Jr., Steven M. Platek, Benjamin C. Ampel, and John P. Towne.