If Christmas leaves you feeling less than amorous, you’re not alone. Whether it’s the stress of last-minute present buying or overconsumption of food and alcohol, interest in sex drops off in the run-up to Christmas, research suggests. However, this is compensated for by a big spike in sexual activity over the new year. Researchers have long noticed differences in birthrates at various times of the year, but whether this was the result of seasonal fluctuations in fertility or sexual activity was unclear. To investigate, Laura Symul at Stanford University in California and her colleagues turned to data from the Clue women’s health app, which included anonymous sexual activity logs from more than 500,000 women in the UK, France, Brazil, and the US. “It’s self-reported, but it is still the largest dataset of real-time reports of women’s sexual activities,” Symul said. The research showed that holidays – including bank holidays and Valentine’s Day – were always associated with a peak in sexual activity. “There was also a very strong difference between weekend and weekdays – people have more sex on weekends,” said Micaela Martinez at Columbia University in New York, who was also involved in the research. “It suggests that having leisure time with your intimate partner facilitates sex.” However, the records indicated that the three days running up to Christmas represented a no-go zone for many women. In the case of younger or child-free women, this could be due to them spending Christmas with their parents, rather than romantic partners, Symul said. For mothers, and particularly working ones, there may be other factors at play, said Dr Kate Boyer, a senior lecturer in human geography at Cardiff University, who wasn’t involved in the study. “Christmas carries a lot of work, and expectations, with it: from organising and wrapping presents, to making the home look different and special, to preparing special foods and perhaps doing Christmas cards,” she said. “In most families there isn’t someone at home who can make this ‘holiday work’ their priority, so it ends up getting squeezed in around jobs and childcare. It just isn’t a recipe for feeling sexy.” Things improved once Christmas arrived, with Clue users reporting a sustained surge in sexual activity that lasted from Boxing Day until the new year. New Year’s Day saw the biggest peak, although because of the way the app works, any sexual activity after midnight on 31 December would also count as New Year’s Day sex. Symul and colleagues compared women’s sexual activity data with official birth records for each of the relevant countries and found a slight increase in birthrates between June and November in northern hemisphere countries, peaking in September in the UK. This pattern couldn’t be explained by seasonal changes in sexual activity alone. But it could, in combination with seasonal variations in human fertility, with this peaking during autumn and early winter, statistical analysis revealed. “We think these two things are acting together to shape the birth seasonality that is experienced in the real world,” said Martinez. The research was published as a preprint and has not yet been peer reviewed. If the seasonal fertility pattern they have identified really does exist, it appears to be relatively weak compared to true seasonal breeders such as sheep or hamsters, said David Ray, a professor of endocrinology at the University of Oxford. “It is probably more of an interesting thing, rather than being medically important.” However, the findings also fit with recent research suggesting that there are seasonal fluctuations in the human immune response. “Although humans perhaps evolved in equatorial lands, where seasonal breeding may not have been as important as to mammals in temperate zones, there may be vestigial seasonal circuits in us, which can be detected if you look in a large enough number of people,” Ray said.