Women were more likely than men to report falling asleep after sexual intercourse, with or without orgams; postcopulatory somnolence was also enhanced by orgasm in both women & men

Gallup, G. G., Jr., Platek, S. M., Ampel, B. C., & Towne, J. P. (2020). Sex differences in the sedative properties of heterosexual intercourse. Evolutionary Behavioral SciencesFeb 2020. https://doi.org/10.1037/ebs0000196

Abstract: Based on a sample of 128 female and 98 male college students, there were significant sex differences in the sedative properties of vaginal intercourse. Consistent with predictions derived from an evolutionary model of sperm retention and human bipedalism, women were more likely than men to report falling asleep after sexual intercourse. 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. Consistent with the possibility that seminal fluid may contain sedative-like properties, 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

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Women who were inseminated were also more likely to fall asleep... that raises the possibility that human semen may contain sedative properties... the so-called missionary position may have evolved to promote sperm retention, and anything that postpones the sumption of an upright posture after insemination would function to increase the likelihood of sperm retention and impregnation as a consequence.