Ah, summer. The season of flirtation, late night drinks on roof decks, and uncomplicated flings. If the warmer weather in the northern hemisphere has brought a renewed interest in meeting sexual partners, then you’re not alone. Numerous data sets have found distinct seasons in sexual interest throughout the year, with summer marking one such sexual peak.

There are several ways of evaluating interest in sexuality, which in the US appears to increase both around Christmas and in summer months. Data on birth rates and abortion suggest that there’s an increase in sexual activity in December. Meanwhile condom sales, STI diagnosis rates, and surveys of when people lose their virginity show a bimodal trend, with peaks around both December and summer.

Patrick Markey, a psychology professor at Villanova University, found a similar trend when he studied Google search words as an indicator of sexual interest. Searches for prostitution, pornography, and dating websites all increased in the summer months.

But it’s not exactly clear what’s behind this uptick in sexual interest.

“We don’t know if it’s some kind of leftover mechanism from our ancestral past that we have this mating cycle, or if it’s something that results from circumstances,” says Markey. “When you see the pattern across so many different data sources it shows, ok, there are these cycles of sex. They’re definitely there, we just don’t know why they’re there.”

It’s plausible that there’s an evolutionary purpose behind mating in the summer, or else annual variations in sperm quality and hormonal fluctuations. But Markey says he leans toward simpler theories, which point toward changes in our environment.

“I like the idea that in summer time, we’re out and about, it’s hot, students are out of school, there’s more sociability going on with less clothing,” he says. The December peak could potentially be explained by just the opposite—“We’re inside, cooped up, what else is there to do,” he adds.

Exotic summer holidays could potentially lead to more romances, or the change in season could act as a marker, much like holidays during December, reminding us that it might be time to start new relationships.

But while the reasons for these sexual surges are unknown, Markey says that the evidence is “consistent.” Though there may not be wild variations in one individual, the trend is clear from surveys with millions of data points.

“In most of science we have a theory and then data to support the theory, and here it’s the exact opposite, we have the data that shows these trends and now we’re trying to come up with a story around it,” says Markey. “It’s data that doesn’t have a known explanation.”

Then again, romance has never paired well with logical explanations.