According to Your Body, There Are Only Two Seasons

New research suggests human biology only knows winter and spring

Image: Xuanyu Han/Getty Images

Autumn leaves changing color. The first winter snow. Daffodil blossoms signaling the start of spring. The long, hot days of summer. At temperate latitudes in Europe and the Americas, nature’s four seasons are a big part of people’s lives. But it turns out human biology has a different schedule.

In a recent study published in the scientific journal Nature Communications, Stanford geneticist Michael Snyder, PhD, looked at how people’s biological data changed over the course of the year. Armed with a vast trove of information — over 1,000 measurements from more than 100 people assessing genes, proteins, metabolic markers, immune system markers, and the microbiome — he discovered that instead of four distinct seasons, the body seems to undergo two shifts: one at the beginning of winter and the other in the middle of spring.

Elemental spoke with Snyder about the recent study, what might explain the biological seasons, and what they mean for your health. This interview has been lightly edited for length and clarity.

Elemental: What inspired you to look at seasonal changes in biology?

Michael Snyder: I had recently been wondering, why do we think there’s four seasons? It’s kind of arbitrary. Maybe there’s 15 seasons, maybe there’s three, I don’t know. Why don’t we let the data tell us how many seasons there are, at least from the standpoint of human biology. Meaning, are there patterns in the data to tell you just how many seasons there really are? So the inspiration for this study was really the combination of those two things: always trying to understand people’s health patterns, and the concept that seasons are pretty arbitrary, when you think about it.

What did you find?

We’ve been profiling this group of 109 people for a number of years, and we had over a thousand measurements in total. We just looked for biological patterns in the data. First, we started with individual molecules, and we saw [patterns] that were known already. For example, hemoglobin A1C [a test that measures blood sugar levels] was known to peak in the spring. We also found a lot of new [patterns], as you might imagine, because we looked at so many molecules, so there’s a lot of stuff that hadn’t been reported. One was [the expression of] this circadian rhythm gene that’s called CIR1 that’s known to fluctuate during the day. But what’s interesting is that we found it actually showed a seasonal pattern and peaks in late April/early May. Then, there were changes in various cytokines [immune system proteins] that are involved in fighting off viral illnesses. We were also very interested in the microbiome, which had been studied a little bit, but again, not at the level we did, and we saw quite a few changes there as well.

After we saw these individual molecule changes, we said, “If we take all these molecules together, do they fall into major patterns?” Turns out, they do. There are two major patterns [in these 109 people]: one is what you would expect, it’s late December/early January — a winter pattern, if you will. But then you might have thought the other pattern should be July or August when it’s really hot — late summer. But that wasn’t the case at all, actually. The pattern that came up was late April/early May. So that was, at least to me, a surprise.

All of the people in the study are from Northern California, and obviously the seasons there are different than in New England or the Southeast. Do you think that the two-season pattern that you found would hold up elsewhere, or do you think it would be dictated by local weather patterns and temperature changes?

I don’t know, but I think the approach we use now can be applied anywhere, we’ve just got to get the data. There’s no reason it has to be two; in some places it might be three seasons, in other places it could be 10. Who knows? I think it would be really fun to figure that out.

What different kinds of molecules did you look at? I know that in prior studies, you’ve analyzed the genome, the metabolome, and the microbiome. Where did you see the biggest differences in this study?

We measure as many molecules as possible. A number of them are clinical markers, but the bulk of them are RNA — your transcripts, proteins, and metabolites. We measure all those. It’s about 20,000 molecules overall, plus microbes.

We saw tons and tons of microbes in people’s nasal cavity changing, plus some changing in the gut. I think that makes sense given people’s exercise and probably the food they eat [change with the seasons], so they should see some shifts in their gut microbiome. Also in their nasal cavity because, again, you’re sucking up whatever’s around you, and that’s going to change through the year.

Fascinating. What do you think explains these seasonal patterns? Is it driven by the biology or the environment?

The winter pattern is what you would expect, it’s viral infections and things like that showing up [and changing the biology]. But there’s some other cool stuff too, [particularly with the microbiome]. Bacteria associated with acne, for example, actually peak in the winter. In the late spring, early April, asthma and allergies [drive a lot of the molecular changes], again which you would expect. But there are also a lot of metabolic changes I didn’t really expect. Our explanation is that people are kind of dormant through the rainy season in California. That is to say, they exercise less than they would in summer. So what we think is that stuff builds up through, say, March and April. Then as people come out of that, they start exercising more, and their metabolic health and cardiovascular health improves. In hindsight, it makes a lot of sense.

Can you talk a little more about the relationship between the environment and people’s health? The changes in the immune system and inflammation that you’re seeing, for example, is that solely driven by the increase in viral infections during the winter, or could that be a change that makes someone more susceptible to a viral infection?

That’s a good question. I think it’s probably in part due to the viral infections, because we’re seeing cytokines go up. We’re only showing associations, so we can’t prove one versus the other, but there’s most likely an increase in viral infections. But it’s true that people are less healthy in winter, so that probably does make them a little more susceptible as well. They probably go together.

The spring pattern is much more complex. It definitely has the allergy and asthma signature, but it also has all those metabolic markers, like hemoglobin A1C, which is associated with diabetes and insulin resistance. Some of the type 2 diabetes markers are high in the spring pattern; same with the cardiovascular disease markers. And again, I think it does relate to the external environment — in this case, we can correlate it with pollen. We haven’t published this yet, but we can definitely see correlations between external exposures and some of the internal metabolites, meaning we see things happening on the outside, like pollen, can correlate with metabolic changes inside of you. So we think that some markers correlate with the environment, and some correlate with people’s lifestyle.

What are the implications of knowing our “health seasons”?

I think the implications are twofold. One is, we don’t really use longitudinal data very well in medicine. One of my biggest gripes is that when you walk into a doctor’s office, they measure you and they compare you with what they’re expecting across the population. They’re not looking for trends, and I would argue that you really should be following people’s trends. For example, if your hemoglobin A1C is going up, you really want to follow that. But it’s nice to know what the seasonal effects are, too. So if something’s running a little bit high in late April/early May, you can say, ‘Well, that’s probably because they’ve been less active and it might be seasonal, but they probably can get it down as they head into summer.’ Now, if it’s running high in winter, then there’s probably something going off there and it’s good to have caught that. So you can take this into account as you’re interpreting people’s health.

Or it could mean that you need to work a little bit harder, so you don’t have a higher peak of cardiovascular disease and metabolic disease markers in late April. Maybe people should be pushing themselves a little bit more during those rainy seasons to keep those markers down. That would be another way to interpret it. I think it helps interpret people’s health and what to do about it.

Health and science writer • PhD in 🧠 • Words in Scientific American, STAT, The Atlantic, The Guardian • Award-winning Covid-19 coverage for Elemental

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