Hormones are seasonal for men and women alike, according to a study of millions of tests from medical records.

The dataset's evidence for hormone seasonality points to a winter-spring peak in hormones for reproduction, growth, metabolism and stress adaptation. Published Feb. 16 in Proceedings of the National Academy of Sciences, the study builds on research showing that humans may have a physiological peak season for such basic biological functions.

"When we think about humans, we don't think that we're seasonals," such that humans are "different animals" depending on the season, study author Alon Bar, a Ph.D. student at the Weizmann Institute of Science in Israel, said in an interview with The Academic Times. "I think that it's surprising to see that so many hormones peak together at the same time of the year."

The research team posits that there may be some physiological explanation behind the winter-spring peak in hormones, which are made in the endocrine glands, for certain human biological functions. 

"The physiological effect of such changes is not clear," they noted. Yet they were still able to pinpoint an exact antiphase, referring to when the activity of hormones changes through the year.

The team found seasonality to be "coordinated with a winter-spring peak in effector hormones and a surprising antiphase between pituitary and effector hormones." In explaining the antiphase, the team pointed to evidence showing that "trophic effects of the hormones create a circuit in which the functional masses of the glands change over the year and can entrain to yearly signals." 

An effector hormone such as oxytocin refers to an organ or cell that responds to a stimulus. A pituitary hormone comes from the pituitary, a small gland on the base of the brain targeting specific parts of the body.

To arrive at the results, Bar, who was in charge of the data analysis for the project, obtained data from medical health providers on millions of blood tests showing hormone seasonality. The dataset has measurements from about half of the entire Israeli population over 15 years, between 2002 and 2017, totaling 46 million person-years of diverse socioeconomic and ethnic representations. 

By analyzing seasonality in blood-test data, the team saw a delay for most effector hormones. It points to Dec. 21 and June 21 as an antiphase, at which point most pituitary hormones peak. The delay shifts the peak activity of most effector hormones from Dec. 21 to later in the winter or to spring.

The team started working on a hypothalamic−pituitary−adrenal axis model, which governs stress response, to see whether there were changes in hormones. The system includes a new aspect of interaction of hormones that causes the glands to proliferate. 

"We get a lot of interesting dynamics," using the multi-model system, Bar noted. "One of the predictions of these models is that the hypothalamic−pituitary−adrenal axis should show specific phases of seasonality in regards to changing photoperiod input." Photoperiods refer to day length.

Besides providing the first predictions of the winter-spring peak in human hormones, Bar further noted that the team developed a model for a so-called circannual clock.

The team's proposed clock keeps track of seasonal phases of hormones based on the time of day. People can use the clock to track changes in circadian rhythms, including the maintenance of photoperiods and temperature conditions. They can reportedly also address circadian rhythm concerns, such as insomnia and depression, using the new clock. 

The study, "Hormone seasonality in medical records suggests circannual endocrine circuits," published Feb. 16 in Proceedings of the National Academy of Sciences, was authored by Avichai Tendler, Alon Bar, Netta Mendelsohn-Cohen, Omer Karin, Yael Korem Kohanim, Lior Maimon, Tomer Milo, Moriya Raz, Avi Mayo, Amos Tanay and Uri Alon, Weizmann Institute of Science.