Action between the sheets has greater sedative properties for women than men, say researchers, adding that women are more likely to fall asleep after sex, some cuddle and pillow talk than men.
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 sexual encounter compared to men.
"Postcopulatory somnolence was also enhanced by orgasm in both women and men. However, with or without orgasm, women were more likely than men to report falling asleep after sex," said study authors from State University of New York at Albany in the US.
The findings also showed that women who were being inseminated were also more likely to fall asleep after sex. There was no evidence for sex differences in the sedative properties of masturbation, though.
"Our paper summarises 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," study author Gordon G Gallup was quoted as saying in a PsyPost report.
"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," Gallup added.
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, the researchers said.