The environment has a significant influence on our overall health, yet we don’t know a lot about the impact of the changing seasons on our biological processes. If we ignored the dates on the calendar, would our bodies still feel the seasons change on a molecular level? Scientists are learning that our bodies do recognize changing seasons, but not necessarily the four seasons we structure our lives around.

In a study published last year, researchers at the Stanford School of Medicine set out to identify seasonal biological patterns based on molecular data, not calendar dates. To do this, scientists conducted an in-depth longitudinal multi-omics profile of patients’ blood over four years. Over 100 generally healthy individuals participated in the study, 55 women and 50 men between the ages of 25 and 75 years old. Researchers collected blood samples from participants four times a year. By closely studying a patient’s blood for multiple years, scientists were able to look for seasonal patterns. The researchers saw that molecules grouped into two major seasons, with peaks in late spring and late fall/early winter.

In essence, the body only recognized two seasons, not four.

An important caveat—this study was conducted in California, which means seasonal peaks likely occur at different times of year in different places. Research from Gambia, for example, found that numbers of seasonal cell types peaked from June through October during the rainy season. In the San Francisco bay area where the present study took place, there are really only two seasons from a climate perspective. A long, comfortably warm, and dry summer is followed by a mild and rainy winter. Researchers note that the two major human biological seasonal patterns they discovered are likely influenced by these climate conditions and the corresponding lifestyle changes. For example, the first peak in late spring happens at the onset of a heavy pollen count and after a period of reduced physical activity (due to more time spent inside)

However, the geographical limitations of this study do not make the results less significant. This study demonstrates that the molecules and microbiomes in our bodies develop their own seasonal patterns in response to environmental changes. In late spring, researchers found that inflammation was extremely high, likely due to more pollen and less exercise. Conversely, in late fall, researchers saw an increase in immune molecules to fight viral infections and higher blood pressure. Understanding these patterns makes it easier to determine seasonal risk for specific diseases and discover new health biomarkers to diagnose and treat symptoms.

Plus, “since every geographical location has its unique climate conditions, our approach can be applied to any geographic allocation around the planet to capture the seasonal human biology associated with these locations,” the researchers explain. In the meantime, we can use the knowledge that our bodies go through seasonal changes to better manage our own health and be kind to ourselves