Med thumb couple sex istock oxytocin story

Sex and sleep are forever entwined. The bedroom, so often the most personal space in one’s home, is the obvious setting for such an intimate act. We don't just sleep next to mates for warmth, after all. Ask any sleep expert about best bedroom practices, and chances are they'll prescribe just two activities — both of which begin with “S.” When someone says they “slept with someone,” you know precisely what they mean.

Sex and sleep just make sense together. A bed is soft and comforting. And, truth be told, we all feel like passing out once the deed is done.

Yet there may be a deeper reason for the association between the two. Dr. Sue Carter, the current director of Indiana University’s acclaimed Kinsey Institute for Research in Sex, Gender and Reproduction, suggests that a hormone released after sex facilitates sleep for biological purposes. A distinguished behavioral neurobiologist who has won multiple awards for her work, Dr. Carter is renowned for her work on stress hormones and how they help foster relationships. In particular, she’s known for groundbreaking research on the neuropeptide oxytocin, also known as "the love hormone."

Dr. Carter believes there are innate reasons why sleep and sex are so tightly connected in the human condition. “This has yet to be sufficiently studied," she told Van Winkle's, but "there’s a lot about oxytocin and sex and its association with an increase in sleep.”

Here are a few of her thoughts.

Oxytocin Calm Down

1. Good sex calms you down

That wave of post-coital bliss? Thank oxytocin.

“The hormone, which is released during orgasm, is associated with reducing stress, reducing defensiveness and fostering immobility without fear. That's the ideal situation for going to sleep,” Dr. Carter said.

“To be able to lie still at night and not having your brain racing and thinking about the things that are bothering you, or that you have to do the next day, is ideal for quality rest.”

2. Sex hormones stimulate repair, a primary sleep function

According to Dr. Carter, oxytocin is released by sexual experience in a big surge — but it doesn’t stay elevated for very long.

“Our bodies then put it out as a kind of pulse, and that pulse can be highly therapeutic. For example, evidence points to oxytocin triggering undifferentiated stem cells to turn cardiomyocyte into new cells to repair the heart.”

Seeing as repair is a known function of sleep, Carter believes there must be kind of association between the two.

“How it works is not understood,” she admitted, “but it has to be part of the story.”

Oxytocin Fertilization

3. If the biological goal of sex is fertilization, immobilization is important

In a normal, safe environment, the oxytocin released from a toe-curling orgasm leads to an increased sense of safety. And, more likely in women, a sense of immobilization.

“The goal, the physiological goal, as far as sex is concerned, is reproduction,” Dr. Carter said. “And that goal is accomplished more efficiently if a woman holds still. In this case, the semen is less likely to be lost, and the sperm have extra time to swim upstream.” 

Dr. Carter further explained that sexual behavior itself may release hormones associated with ovulation; the act of having sex may trigger the egg to be expelled from the ovary, into the oviduct. This is called "induced ovulation," and it's common in many species.

“What you have with the biology of oxytocin are some mechanisms that are going to facilitate fertilization,” Carter says. “And that facilitation would include immobility and, possibly, sleep."