Ancient Egyptians identified three seasons based on cycles of the Nile River: inundation, emergence, and harvest. In tropical countries, it’s the rain that tends to divide the year in two: a wet and a dry season. Elsewhere the calendar demarcates four seasons: autumn, winter, spring, and summer.
But now, researchers at Stanford University have found that human biology, rather than rivers, rainfall, or calendars, could be used to determine the seasons. In their study, published in the journal Nature Communications in October 2020, the Stanford researchers discovered our bodies seem to set their own rhythm, splitting the year into two seasonal time periods. Or at least that’s the case if you live in California, where the study was carried out. Since every geographical location has unique environmental conditions, their approach may be used the count the seasons in other parts of the world too.
“People say there are four seasons of three months each. But why four? There could be 15 or could be 2. Why don’t we let biology tell us?” asks Prof Michael Snyder, principal investigator of the study.
To determine the human seasons, Snyder’s team profiled the biology of 105 volunteers in the San Francisco Bay area over a period of four years. They regularly sampled and measured tens of thousands of molecules and microbes from the participants’ blood, noses, and guts. This type of study is called ‘deep longitudinal multi-omics profiling’.
On sample days, the researchers also collected meteorological data (such as air temperature and solar radiation) and airborne pollen counts.
This massive effort was undertaken to create a better picture of how the changing seasons might be affecting our physiology and health.
MAKING SENSE OF THE SEASONS
After four years of testing poo, taking blood samples, and logging the weather, the team used powerful computational tools to try and find patterns between the volunteers’ biology and their environment. What they found surprised them.
There were two signals. One was a group of molecules that seemed to peak in December – a season the researchers dubbed late fall/early winter. This included markers related to immune responses such as the complement system, a collection of proteins that work together to eliminate infectious microorganisms, which peaked during this time. Unsurprisingly, this correlated with the period we know viral infections are also high. The second signal however did come as a surprise.
“I thought the other [season] would be in June or July when it’s pretty hot, but that wasn’t true,” Snyder says. Instead, the second season peaked in late April – a season they called ‘late spring’. This season’s peak made sense in hindsight, as late April also corresponded with a time of high pollen counts at the end of California’s rainy season. The pollen caused a reaction in a large enough subset of people to contribute to the seasonal peak in the immune response.
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