Could ‘Season Creep’ Affect Human Behavior?

RACHEL FELTMAN: This is Science Friday. I’m Rachel Feltman.

Maybe you’ve noticed lately that the seasons just don’t quite behave like they used to. I’m an autumn girlie myself. I love the crisp weather and the spooky vibes, and these days, fall just seems to happen in the blink of an eye if it even happens at all. Spring flies by in a flash, too, while winter seems more erratic and intense, and summer goes on forever.

This shift in the timing of the seasons is often called season creep. This phenomenon is mostly due to climate change creating temperature imbalances and encouraging wacky weather patterns year round, and it can cause problems for plants and animals as their natural cycles fall out of sync. But what does it mean for human behavior?

It’s no surprise that the seasons can affect our mood. You’ve probably heard of seasonal affective disorder, often called seasonal depression, which tends to hit during the cold, dark winter months. But as the seasons start changing more quickly and unpredictably, the shift could have a wide range of effects on us that we’re only just beginning to understand.

Joining me today to try to answer some of these questions is Dr. Michael Varnum, social psychology area head and associate professor at Arizona State University. Welcome to Science Friday. Thanks so much for joining us.

MICHAEL VARNUM: Hi, thanks so much for having me, Rachel.

RACHEL FELTMAN: So you and your colleagues recently wrote a paper on the topic of human seasonality. Could you explain what that means and why it’s an important topic to unpack?

MICHAEL VARNUM: Sure thing. So we got interested in this question of when we look at many other non-human animals, if we think about brown bears and squirrels that hibernate in the winter or Canada geese which fly south in the fall in search of warmer weather, we look across the animal kingdom and we look at plants as well, and we see a wide range of seasonal effects on their physiology and behavior. And when we think about ourselves, most of us are aware of some of these seasonal shifts in mood, but, in fact, it turns out that we’re fairly seasonal creatures as well.

RACHEL FELTMAN: Yeah. So what are some of the ways that seasons can influence human behavior?

MICHAEL VARNUM: A lot of things seem to follow seasonal patterns in our species. So it’s not just this tendency to increase feelings of depression in wintertime. Recent work by some of my collaborators actually suggests that anxiety seems to peak in spring and fall, and we also see changes in things like aggressive behavior.

So we’ve known for some years looking at crime statistics and other sources of data that people get a little more violent and aggressive in the summertime, but it also turns out if we look at generosity, that tends to peak in the wintertime. There’s a bump in charitable giving and even how much people leave is tips at restaurants around the Christmas holidays. And beyond that, we see changes in everything from the kinds of foods we eat and levels of exercise to our preferences for different kinds of colors and even our sexual behavior.

So it turns out in the summertime and also again around the holidays, a variety of sources of data looking at large surveys of people’s frequency of sex as well as internet searches for things like pornography and condom sales, these seem to show biannual peaks as well.

RACHEL FELTMAN: Wow, that’s so interesting. Do we have any idea why seasons can affect us in these ways?

MICHAEL VARNUM: Yeah. So some of these changes probably have to do with the way amounts of sunlight and heat affect our neurotransmitters and our hormones. There’s emerging evidence suggesting that serotonin tends to start to bottom out in the wintertime, and sex hormones like estrogen and testosterone, according to large analyzes of medical records, seem to have biannual peaks in the winter and summer as well.

But more broadly, we also see when we think about seasonal changes, it’s not just the temperature or whether we’re getting snow or how much light we’re getting that changes, other aspects of our environment that have been shown to affect our behavior also shift. So if we think about the wintertime– this is cold and flu and now COVID season– so the threat of infectious disease tends to follow cyclical patterns, and that’s been linked from everything to our tendency to conform to how willing we are to try new things to whether folks are suspicious of people who are unfamiliar or don’t look like them.

We also see shifts, for example, in places like Arizona where I live. We get influxes of guests and tourists in our winter months because it’s a lot nicer here than, say, up in Canada or the Midwest. And so the density of population can shift seasonally, and that can affect the extent to which we engage in long-term planning. And in addition to this, seasons also have cultural associations. Christmas, Hanukkah, a lot of other holidays fall in those December months, but we also see– we can think about the cycles of the school year and other ways in which our lives are structured in these seasonal patterns. And likely some of these effects that have been observed are due to a complex interplay of these factors.

RACHEL FELTMAN: And it’s interesting that you mentioned respiratory disease spikes. I know there’s been a lot of talk this summer in particular about COVID spikes in the summer, and I’ve seen some public health experts speculate that maybe as summers are getting more oppressively hot and people are spending more time indoors in the AC, the traditional wisdom about viral transmission being lower in the summer because everyone’s enjoying time outside might be less of a truth these days. Do you have any thoughts on other ways that climate change and season creep could be impacting those effects on our behavior, and is anyone researching that specific question?

MICHAEL VARNUM: Yeah, those are great questions. So one of the big effects of climate change aside from season creep is that our summers are not only getting longer, but they’re getting hotter. And there’s emerging research on what do extreme high temperatures and exposure to them do to our health and our psychology.

A recent review, this was by Rony and Alamgir, found that we get more irritable when we’re exposed to these really hot temperatures, which makes sense but also that our sleep tends to be disrupted. We tend to experience decreased motivation and increased feelings of helplessness, which living in the Phoenix valley when it’s been above 110 for a month and a half straight sometimes, yeah, we do feel pretty gloomy about life in general and like we’re doomed. So a lot of things might be responding to this increase in extreme heat.

RACHEL FELTMAN: Yeah, absolutely. I know in the New York City area, we had a brief false fall and then normal August weather. And now we’ve crept back up into heatwave territory. And I am definitely feeling less hopeful. But I– I hope that autumn is still in sight.

So how can seasons happening earlier or later disrupt our behavior patterns?

MICHAEL VARNUM: So there’s some research emerging for other species. If we look at migratory birds, over the past few decades, they’re actually starting to shift those migration patterns by about five to 10 days over the past few decades. Animals that hibernate are arising from their stupor about two weeks earlier than they were earlier in about half a century ago or so. Insect populations are emerging and hitting their peaks later. And so, again, it’s not just these other animals that are likely to be affected by these shifts in seasons and their timing, but a lot of the effects we talked about just a few moments ago in humans, we might expect the timing of those to begin to change as well.

RACHEL FELTMAN: Yeah. And what about the actual shift from season to season itself? I know a lot of folks in my area feel like we go right from summer heatwaves that last way longer than they used to, and then suddenly it’s dark and it’s cold and we haven’t had that autumn time to recover. How might that impact our behavior based on what we know about human seasonality?

MICHAEL VARNUM: I think that’s a fascinating question. So we might expect that some of these effects that we see in terms of psychological or behavioral changes that seem to be strongest in spring and fall, I guess they might become muted. So, for example, it turns out studies of college students suggest that working memory is best in the fall and worst in the spring. But maybe you’re going to see more year round stability in some of these cognitive effects.

I’m not entirely certain, but I think these are fascinating questions for us to begin to look into and track and maybe things that with enough archival data, climate change isn’t exactly new. We may be able to look back and see evidence of shifts in the strength or timing of these peaks in things like aggressive behavior or changes in mood or changes in our memory and cognition.

RACHEL FELTMAN: And as we touched on earlier, one commonly known psychological effect of season shifting is seasonal affective disorder. So could you talk a little bit about what that is and why it happens?

MICHAEL VARNUM: It seems like as best we can understand it, our bodies seem to respond to the amount of sunlight we’re exposed to. And as the days shorten, we’re exposed to less of it, and we seem to make less serotonin, which appears to have effects on our mood. For some people, that leads to clinical levels of depression. But large-scale surveys and also analysis of data from what people are posting on social media and what they’re searching for suggests that actually for a lot of people, we see these decreases in mood in the winter time.

Scientists have speculated a bit about why might this be. Is there perhaps some adaptive property of why we get a little sad in the winter time? And it actually– if we step back from it, there’s a lot of parallels to what happens in hibernating animals. And so for most of our history, we didn’t have very good climate control. We didn’t have electric heat or air conditioning. We didn’t have grocery stores, so food might have become scarce in the winter.

So it would have been adaptive, at least in latitudes where you really had these very cold months where not a lot of stuff would grow for us to conserve energy, and this might explain the decrease in motivation. It might explain why we’re moving and exercising less, why we’re eating more and packing on the pounds.

When we’re feeling happier, we tend to be more energetic. We burn more calories. And so this might in some ways be a holdover from most of our evolutionary history.

RACHEL FELTMAN: Yeah, that’s super interesting. On my podcast The Weirdest Thing I Learned This Week, we’ve talked a few times about the idea of second sleep. Before electric light, a lot of people, a lot of cultures had these two phases of sleep, and it was like the sun’s going down, so why would you stay up? But you’re getting more sleep than you need, so you wake up in the middle of the night. You do a couple of things with your one candle and then go back to bed.

And I remember reading some scholars from the time actually said that the best time to try to conceive a child was between your two sleeps because your energy would be at its highest level. So, yeah, I love thinking about just the ways that our routines have changed so much now that we can control when we have light and how warm our house is. And, yeah, I think it’s important to remember that maybe our bodies are really trying to help us out by getting us to slow down in the winter.

But what about the other side of the coin? What are some positive impacts or benefits that seasons changing can bring us?

MICHAEL VARNUM: Well, if we look at the data that Ian Holm and Mark Schaller analyzed on anxiety, I was surprised to read these results. But that anxiety seems to be peaking in fall and spring when they look at the annual cycles of it. And so maybe we’ll get less anxious if we have less fall and spring, I don’t know.

RACHEL FELTMAN: Interesting. And how would you suggest that people prepare for seasonal mood changes?

MICHAEL VARNUM: So I think it may be helpful for a lot of people to realize that these are natural cycles we go through in mood. I imagine in the therapeutic context, knowing that maybe what’s going on in terms of your feelings isn’t entirely due to something idiosyncratic in your life that’s awful or oppressing or depressing but that this is part of a wave that goes up and down and in a regular fashion and that it’s likely to pass as the sun comes creeping back out again. I’m not a clinician, but I imagine knowing that would probably be somewhat helpful.

We also know because in this modern world where we have all these conveniences, we can purposefully engage in more exercise, which is one thing that’s been shown to be really, really effective at improving our mood, reducing feelings of depression and anxiety. So maybe knowing that this is coming, that it’s natural but also proactively taking some steps to do things that we know will improve mood seem like they might be good strategies to deal with these seasonal fluctuations in how we’re feeling.

RACHEL FELTMAN: Absolutely. And how can we prepare for seasons getting less predictable in the future with climate change?

MICHAEL VARNUM: That’s a tough one. In one line of work I’m interested in looking at long patterns of societal and psychological change and thinking about how we can maybe forecast the future of these. So we do have good meteorological and climate data. We have decent archival data going back at least long enough to detect some of these seasonal effects. So I imagine if we can model what these seasonal– how the seasons will change going forward, we might also be able to make some useful forecasts for things like birth rates and reproductive behavior, rates of crime, changes in mental illness symptomology that should be helpful for policy makers, those in the health care industry, folks trying to decide how to allocate resources to deal with these issues.

RACHEL FELTMAN: Yeah. Well, and I suppose there are parts of the world that already have a lot less seasonal change than I’m used to having grown up on in the Northeast. So maybe I can pick up some tips from folks in Arizona and California. What would you like to see researched or studied about this in the future?

MICHAEL VARNUM: Well, I think seasons are an interesting question in the behavioral sciences. It’s one of those where it’s such a ubiquitous part of life that we’ve taken them for granted and not really thought in too concentrated a way about what kinds of effects they may have. And so it’s pretty common for psychologists now to report where in the world they gather their data and how old their participants were and what their demographic backgrounds are because we know this stuff matters. But we don’t really pay attention to when we conduct our research.

And if seasons affect this wide range of behaviors and mental states, everything from mood to sexuality to aggression, then it might be important to start paying a little more attention to that and also to be thinking about can we make some predictions for how things might be a little less obvious might change over the course of the year?

My colleagues and I, Mark Schaller and Ian Holm at UBC, we’re actually running some big longitudinal studies right now looking at whether there might be seasonal shifts in things like xenophobia or seasonal changes in our orientation to individualism and collectivism or how motivated we are to find new romantic partners versus take care of family. And hopefully in another couple seasons, we’ll have some answers for you.

RACHEL FELTMAN: Yeah, absolutely. I can’t wait to read the actual scientific research on cuffing season as TikTok calls it. Thank you so much for joining us. This has been super interesting.

MICHAEL VARNUM: My pleasure. Thank you so much for inviting me on today.

RACHEL FELTMAN: That was Dr. Michael Varnum, social psychology area head and associate professor at Arizona State University, Tempe, Arizona.

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