Human beings are one of the few animals that can engage in sexual activity completely contingent on external environmental factors. This means an overwhelming majority of nonhuman animals reproduce based solely on biological instinct. If we have the freedom to have sex whenever we want, without being naturebound by innate urges to procreate; then why does research show we typically tempt our partners at night?

There has been a wealth of research conducted over the last 30 years, many of which point to the same systematic timing of sexual routine. The proclivity of intercourse, whether for pleasure or for procreation, spikes on Saturdays and Sundays, and then makes another although smaller increase at 6:00 a.m. on Monday mornings.

The first research was conducted in 1982 by John Palmer, Richard Udry, and Naomi Morris, researchers at the University of California, who found an overwhelming majority of people have sexual encounters at night, with a smaller group engaging early in the morning. Lead researcher Palmer and his colleagues collected a data set of 78 young, married couples over a 12-month period and observed their weekly sexual activity routines. The authors found "a rather constant copulatory rate during weekdays, with a large increase on weekend."

When researchers discovered time and time again a daily rhythm indicating sexual encounters overwhelmingly occurred at night, they realized the human construct of a week suggests there is a social component that influences when we do and do not have sex. In fact, 58 percent of all sexual encounters were found to happen at night. But to what extent are our sex schedules determined by our societal conveniences, and to what extent can they be attributed to biology?

In nature, many different species' reproduction depends on their gonadal hormones, which are controlled by the reproductive organs and essentially dictate peak mating time for a female's ovulation cycle. But for humans, it's very different and completely contingent on the environmental factors, such as the social constructs of a weekday, alcohol consumption, societal acceptance, and personal preferences. The variations in hormone levels for humans can greatly affect their ability to both desire sexual engagement and to perform sexually; as a result, successful sex can happen anywhere and at anytime. However, the numbers contradict our freedom.

Humans and primates are still bound by certain societal laws. If a primate has sex at the wrong time of day, they can become vulnerable to attack and predation. While human beings cannot engage in sex in public areas or in front of children, both instances will make them vulnerable to arrest, incarceration, and social ostracism.

Also, because humans engage in sex for reasons other than reproduction, there is a fear of pregnancy, especially when the cost to raise offspring is so high in today's society. As far as the findings of research and observation, humans are the only species that will actively avoid pregnancy and recognize it as a consequence of sexual activity, instead of a means to transmit genetic information to offspring.

When University of South Carolina research biologist Roberto Refinetti wanted to replicate the original findings of Palmer and his team, he found the same nighttime peaks on Saturdays and Sundays around 11 p.m. to 1 a.m., with a 6 a.m. Monday peak. He also uncovered the reasons people believed they practiced nocturnal sex engagement. Thirty-three percent of study participants said their work and family schedule dictated their sexual experiences. Feeling sexual came in second place at 28 percent, while mate availability came in third at 23 percent, and lastly 16 percent found they performed sex because they were already in bed.

Sex drive is also somewhat determined by social constructs, moreso than biological hard-wiring. Men, across all cultures, reported a higher sex drive and less restricted sexual attitudes than women in a 2005 BBC study. Although these findings weren't as surprising as most would think, it's really just the individual's perception of his or her own gender and how sexual he or she should rank among the masses. Men believe they should be the sexually dominant of the two genders and therefore report their libido with overconfidence, while women believe they should show reservation in their sexual escapades and desires, thus underestimating their sexual demands.

So what is thought of as a scheduled sex session could very well just be a social effect, instead of a primal desire to engage in the pleasure of sex.