Humans can have sex anytime we damn well please – so why do we mostly do it in the dark? Here's what science has to say about our preference for nighttime hookups.

The When

Lots of studies have looked at the timing and frequency at which humans tend to have sex, over a range of cycling time windows. For the sake of simplicity, we're going to focus on the daily and weekly rhythms observed in two such studies.


The first was conducted by researchers John Palmer, Richard Udry and Naomi Morris, and published in a 1982 issue of Human Biology. Palmer and his colleagues analyzed the sexual activity of 78 young, married couples over a 12 month period, and observed a distinct weekly rhythm to sexual activity, which the authors note is characterized by "a rather constant copulatory rate during weekdays, with a large increase on weekends." The following graph is adapted from their findings:

The researchers observed a daily rhythm as well, which was characterized by a major evening peak – encompassing 58% of sexual encounters – and another, smaller peak in the morning.


Adherence to the human construct of "a week" suggests there's a strong social component that impacts when we do or do not have sex. And yet, the fact that people like to get busy according to a daily rhythm suggests that sexual activity could be dictated, to some extent, by our biology. "As someone who studies circadian rhythms, I know that practically all functions of the human body exhibit [daily] variation," says University of South Carolina biologist Roberto Refinetti in an interview with io9. "It would be reasonable to expect that 'horniness' would exhibit [biological] rhythmicity."

In 2005, Refinetti sought to reproduce the daily rhythm findings of Palmer and his colleagues in a sample of people with a wider age range, while also probing for possible environmental explanations for their sexual rhythms:


His findings – which revealed a peak in sexual activity at bed time and a second, smaller peak around 6:00 a.m. – matched incredibly well with those of Palmer's team. These times coincided with the average sleep and wake time of the subjects, which Refinetti points out are also "well within the range of bedtimes and wake times observed in various societies around the world."

These studies bring us back to our original question – Why night? – while raising another: to what extent are our sexual exploits determined by society and culture, and to what extent biology?

The Why

For many species on Earth, and most mammals, the ability to partake in intercourse is dictated by the periodic release of gonadal hormones. But humans (and primates, in general) are different. There is plenty of evidence that variations in hormone levels can affect humans' interest in sex – but our ability to engage in sexual congress, to actually perform sexually, has been separated more or less entirely from hormonal control. As a result, we can have sex basically anywhere and anytime we want. And do we ever.


Or rather, we do to a point. Social context and cultural conventions have a way of dictating when we primates get down and dirty. If you're a resus monkey, for example, sex at the wrong time of day can make you vulnerable to predation. If you're a human, having a hump at an elementary school playground on a Tuesday morning makes you vulnerable to arrest, incarceration, and inclusion in the national sex offender registry.


That is to say: there exists a plethora of potent motivating factors that keeps most of us from riding eachother like rabbits while we're, say, riding public transit. And so, generally speaking, we do the dirty in private, away from the prying eyes of fellow humans.

This choice – the choice to bone with abandon, but only where and when we want to – highlights an important aspect of human nature, namely our ability to postpone, plan, and rationalize something as ostensibly passionate and impulsive as having sex.

Take fear of pregnancy, for example. As psychologist Kim Wallen notes in his overview of hormones and sexual motivation in primates, "humans are, as far as we are aware, the only species that actively avoids pregnancy and recognizes pregnancy as a consequence of sexual activity." (Some species, like Gelada monkeys, have been known to abort an unborn fetus to preserve their evolutionary fitness, but there is no evidence that they do so consciously.)


In the end, says Wallen, the role of hormones and other circadian factors in the interaction between sexual desire (i.e. when we want to have sex) and sexual intercourse (i.e. when we actually have sex) is probably to increase "motivation" ("horniness," as Refinetti put it). That being said, "this increased motivation may be insufficient to overcome other motivating factors," like avoiding pregnancy, or social ostracism. I follows, then, that having sex at night may be an emergent feature of our social structure. Generally speaking, you're not having sex if you're eating breakfast, driving to and from work, or making dinner. Maybe the reason we have sex at night is because it's convenient.

And, in fact, Refinetti's findings corroborate this hypothesis. In a followup study to his circadian sex investigation, test subjects were administered a short survey with two main questions. What time of day do you usually have sex? and Why do you have sex at these times (as opposed to other times of day)?


Answers could be written in for both questions, but the following ready made answers were provided for the latter:

  • I feel more sexual at these times.
  • These are the only times when my partner is available.
  • My work/family schedule does not allow me to have sex at other times.
  • I'm already in bed, so why not have sex?

"I concluded that this choice of time is simply due to convenience — that is, that's the time when the two people are in bed and lying close together, so they go ahead and have sex," says Refinetti. A biological explanation for human sexual rhythms may well exist, he says, "but I would expect the effect of circadian rhythmicity on sexual desire to be weak."


"To study it, we would have to keep couples alone in constant darkness for many days and check when they initiate sex," he muses.

Sounds like a sexy study.


The fact that human intercourse is boring – compared to the carnal exploits of the rest of Earth's kinky little creatures, anyway – has kept pricisely no one from discussing and dissecting our sex lives into utter oblivion. And so, over the years, much attention has been paid to the various ins and outs of coitus. A small sampling of recent reportage:


Why do we have sex? Where do we have sex? What kind of sex are we having? How often do we have sex, how often do our friends have sex, and is anyone anywhere having anything close to the right amount of it? Do men have higher sex drives than women? Female libido: would a pill that enhances it tear society apart at the seams, or might it actually save the practice of monogamy?