When Seasons Change, We Do, Too

When Tyler Stevenson was a child, he was fascinated by the seasonal rhythms of his native Ontario, amazed that some animals knew when to leave for the winter while others stayed and adapted in order to survive. Little did he know that fascination would turn into a career that would advance understanding of how the environment shapes human, animal, and plant behavior. Stevenson’s research has shown that seasons affect far more than whether birds (and people) go south for the winter. Seasons impact core biological processes like hormonal fluctuations, with implications for everything from how nighttime smartphone use can disrupt sleep to how climate change can affect the annual patterns of agriculture and disease.

Stevenson, who is a Senior Lecturer in biological sciences at the University of Aberdeen, has shown that seasonal rhythms operate a bit like daily circadian rhythms, which regulate our sleep and appetite. On a daily basis, our brains integrate information from our surroundings, especially from light levels, and trigger the release of certain hormones like melatonin, which is involved in sleep and wakefulness. When the environmental cues are altered – like when we expose our brains to high levels of artificial light at night – circadian rhythms become disrupted. That can affect sleep, so doctors warn people not to use their smartphones right before bed if they want to get a good night’s rest.

Stevenson believes that our modern tendency to create patterns of “endless summer” with artificial light are also affecting our natural seasonal rhythms. On an annual scale, environmental cues like light and temperature trigger seasonal rhythms that affect hormones and therefore physiological processes like reproduction and immune function. Stevenson has documented these patterns in birds, hamsters, and other animals. For example, many animals are more susceptible to illness in the winter, so when their bodies detect that winter is coming (because, for example, the days get shorter), immune function increases. When we alter the natural seasonal cues with artificial light and heating and cooling, we change our bodies’ ability to trigger those processes.

Unlike with circadian rhythms, we don’t really understand how the seasonal rhythms influence us. Stevenson is working to change that. “There is very much a black box of what happens in the brain that leads to the physiological response. I’m trying to unravel what that black box is,” he explains. So far, Stevenson’s efforts have identified hormones that appear to be involved and, fascinatingly, shown that seasonal cues can alter gene expression – that is, even if we are genetically predisposed to certain behaviors, seasonal cues can affect whether or not we enact them. For example, there is evidence that people may be genetically predisposed to be early risers or night owls, and Stevenson believes “it’s entirely possible that this is an environmental process” influenced by cues like the amount of light in our days.

One of the most eye-opening implications of Stevenson’s work is that climate change is likely impacting the seasonal rhythms that have shaped plant and animal behavior for thousands of years. Because rising temperature levels create an “endless summer” phenomenon, all kinds of patterns are changing or are likely to change in the near future. For example, animals tend to reproduce at certain times of the year, due to environmental cues. Since the cues are changing, the breeding and life cycles of some songbirds and their caterpillar prey no longer match, making it more difficult for the birds to survive. An increase in the lifespan and reproduction of pests that infect crops threatens food production. And some diseases, like Lyme disease and Cholera, may become more common in certain regions because they are passed more easily in warm, humid environments.

Considering these environmental impacts is one of the ways that Stevenson is beginning to expand his work in animal species to conduct more work on humans. “Humans are complex,” he says, explaining why there has been limited work to date on their annual patterns. But, he adds enthusiastically, “it’s a scientific challenge,” one he is up for tackling.


tyler_stevensonTyler Stevenson is a recipient of the Federation of Associations in Behavioral & Brain Sciences (FABBS) Early Career Impact Award, which will be presented at a Society for Behavioral Neuroendocrinology event this Fall.