By Andrea Hargraves 
For a long time, the idea persisted that men were the only ones who could sleep right after sex. However, it seems that things are not quite like that.
A study published in the scientific journal “Evolutionary Behavioral Sciences,” in which 296 university students participated (128 women and 98 men), concluded that, in the end, they tend to sleep first.
The phrase “sex is a natural tranquilizer” is often used to express the idea that sexual intercourse can have calming properties, but there are not many studies on this effect.Researchers from the State University of New York at Albany, USA, say. “We found that post-coital sleepiness is increased by orgasm in both sexes and that women are more likely to report post-coital sleepiness.”
the reason? Because it increases the chances of pregnancy. According to evolutionary psychology, humans developed an upright posture and bipedal movements as a way to navigate the environment efficiently and optimally. One disadvantage of this position is that it places the female reproductive system at a downward angle relative to gravity. This is not ideal for retaining sperm and thus increasing the chances of the egg being fertilized by the sperm.
To overcome this problem, evolutionary psychologists say that the “missionary stance” has become universal across cultures. Furthermore, they argue that there is another mechanism by which evolution has increased the chances of pregnancy: endowing semen with tranquilizer-like properties. This encourages the woman to lie down after sex, which allows more sperm to be retained in the reproductive system and increases the chances of conceiving.