Sometimes you have a sex question that's not just, you know, an idle passing thought. And in those times you need a real answer—one that's based on deep research and scientific rigor. In those times you need Hard Data.

In the world of online dating, the single busiest day each year is the Sunday after New Year's Day, at least according to press releases issued by dating sites like Match.com. They even have an official name for it: "Dating Sunday." You can think of it as the romance industry's equivalent of "Black Friday."

Most news stories on this subject chalk the phenomenon up to nothing more than people seeking a fresh start. Indeed, as you well know, many people make resolutions at the beginning of each new year after reflecting on their lives and thinking about what they want to do differently. For singles, perhaps this means resolving to find love.

Although this explanation would seem to make a lot of sense, it's actually only one small part of the story. There's far more to "Dating Sunday" than meets the eye.

Let me start by saying that scientists have indeed confirmed what Match.com and other dating sites have reported about this spike in winter interest: A 2013 study published in the Archives of Sexual Behavior revealed that Google keyword searches for online dating companies (like Match, EHarmony, and OK Cupid) reliably increased during December and January over a five-year period. This tells us that "Dating Sunday" is really just the pinnacle of a more extensive winter surge in online dating.

But it's not just that—the study also revealed that Google searches for both pornography and prostitution increase reliably at the same time. In other words, there seems to be a broad, seasonal increase in sexual desire during the winter months, and all the interest that dating sites are seeing is just one small manifestation of it.

What accounts for the increased interest in sex in the winter? We can't say for sure, but one possibility is that it's driven by hormones. For example, longitudinal research has found a seasonal winter peak in men's testosterone levels. What we're seeing here could be just a function of the natural fluctuation of hormones. Another non-biological possibility, of course, is that all of the winter holiday festivities simply get people in the mood for swapping more than gifts.

In addition to seasonal changes in sexual desire, there's something else that is likely playing a role here, too: Many people experience strong emotional shifts in the winter.

A 2005 European study found that about 1 in 5 male and female participants experienced significant seasonal variations in their mood and behavior, with the biggest changes occurring during the first few months of the year. Those who experienced more of these changes reported more mental distress, including feelings of depression and loneliness.

Compared to people in relationships, singles were more likely to report these seasonal changes in mood. So part of what might be driving Dating Sunday and the broader seasonal rise in online dating sign-ups is that the winter blues tend to hit singles harder.

If you're wondering what's causing these changes in mood, scientists believe it has to do with levels of the neurotransmitter serotonin, a key brain chemical involved in the regulation of mood—and the one many antidepressant drugs, like Prozac, aim to increase.

Researchers have found that the rate at which serotonin is produced in the brain is related to the amount of sunlight we're exposed to, with less sun meaning less serotonin. Given that the days are shorter in the winter, it makes sense that serotonin production might be lower at this time of year.