Cracking the Code: How Seasons Shape Your Metabolism

Seasonal Metabolism


Do you feel like your skinny jeans are tighter come winter? You’re not imagining things! Researchers at the University of Copenhagen have discovered that our metabolisms slow when the days get shorter.

The researchers took a close look at how seasons and light exposure impact our calorie-burning engines. What they found will make you want to soak up some sun! As the length of days changes throughout the year, so too do our metabolic processes, ebbing and flowing in rhythm with the seasons.

The less daylight we get, the more our metabolisms slow to a crawl, hanging on to calories instead of burning them for fuel. But during the long bright days of summer, our bodies kick into high gear, blazing through calories to keep pace with all that reviving sunlight.

Decoding the Impact on Energy Metabolism

The most fascinating part of this research is how the number of daylight hours can impact our eating habits and how many calories we burn. For a long time, we’ve assumed that summer, with all its sunshine and vitamin D, is the healthiest time of year. But this new study with mice suggests winter might give us a boost metabolically – at least for some of us.

The researchers exposed laboratory mice to different light hours, replicating the variations between summer and winter. Meanwhile, they meticulously tracked the metabolism and weight changes of mice exposed to “winter light” conditions with fewer daylight hours, versus “summer light” with more daylight.

What they found was surprising. As lead researcher Lewin Small described, “We saw big differences between summer and winter in terms of the mice’s energy use, body weight, fat stores – even their liver fat was affected by the seasonal light differences.”

So, while summer still feels rejuvenating, this indicates that the winter months don’t necessarily put us at a health disadvantage the way we might assume. The seasonal shifts in daylight trigger metabolic changes like what these mice experienced.

It makes you stop and appreciate the way our bodies sync with the rhythms of the natural world around you. More research is needed, but these discoveries challenge old assumptions and give us new insights into the health impacts of changing seasons.

Illuminating the Seasonal Disparities

What sparked this research? It was noted how much daylight hours can vary between summer and winter, depending on where you live. Intrigued by how light exposure changes impact us metabolically, the researchers did experiments with lab mice, mimicking the longer and shorter days.

What they discovered was fascinating – even in mice, which don’t hibernate, or anything based on seasons, there were measurable effects on their metabolism just from the change in daylight hours. Markers like body weight, fat stores, and liver fat content shifted back and forth based on whether they were under “summer” or “winter” light conditions.

So even though we don’t think of mice having seasonal rhythms biologically, it turns out light exposure alone can nudge their metabolism in different directions. This means our human bodies are even more sensitive to the ebb and flow of daylight through the year.

Bridging the Gap: From Mice to Humans

While this study was carried out on mice, it points to an important discovery – that differences in daylight hours can affect how our bodies metabolize energy. As professor Juleen Zierath from the Novo Nordisk Center explains, “What’s fascinating is that this effect translates to humans as well.”

As she points out, further research with people may show that tweaking our artificial or natural light exposure over the course of the year could boost our metabolic health. So, changing when we turn lights on at home or trying to get more daylight in the darker winter months – could nudge our metabolism in a healthier direction.

Like overextending a spring, nighttime light appears to wind up our metabolic machinery too tight when it should be unwinding as bedtime approaches. Over time, this unwinds our ability to regulate blood sugar properly, potentially increasing susceptibility to diabetes.

The mice study reminds us that we humans are likely sensitive to these seasonal light shifts too, even if we don’t hibernate! Our bodies synchronize with natural cycles more than we realize. So, we should pay attention to how light exposure changes through the seasons where we live.

Simple adjustments responding to winter darkness versus summer sun could make a real difference in managing weight and metabolic wellness. We need more research, but it opens exciting possibilities for using light to optimize our health.

In humans, we know that artificial light exposure at night negatively affects metabolic health. As the sun sets each evening, many of us flip on lights to brighten up the night. However emerging research reveals that artificial light exposure after dark sends our metabolism into a tailspin.

Scientists have discovered that evening light can throw off our body’s delicate systems in ways we might not expect. Exposure to bright light at night, whether we’re awake watching TV or even sleeping with a light on in the room, seems to impair how our bodies process sugars and respond to insulin.

A Glimpse into the Future

What does this mean? Optimizing when we expose ourselves to natural and artificial light over the course of the year could help improve our weight regulation.

Changes in light throughout the year could also influence our hunger signaling pathways and when we feel hungry during the day. So, this research gives us new insight into the profound ways light exposure patterns may sync our bodies’ rhythms to seasonal cycles. It makes us reconsider assumptions about how seasons do—or don’t—impact our wellbeing.

As we unravel these mysteries, we better understand how aligning lifestyle factors like light exposure with our innate biological clocks could optimize health outcomes.


  • Lewin Small, Leonidas S. Lundell, Jo Iversen, Amy M. Ehrlich, Morten Dall, Astrid L. Basse, Emilie Dalbram, Ann N. Hansen, Jonas T. Treebak, Romain Barrès, Juleen R. Zierath. Seasonal light hours modulate peripheral clocks and energy metabolism in mice. Cell Metabolism, 2023; 35 (10): 1722 DOI: 10.1016/j.cmet.2023.08.005.
  • Mason, Ivy C, Daniela Grimaldi, Kathryn J Reid, Chloe Warlick, Roneil G Malkani, Sabra M Abbott, and Phyllis C Zee. “Light Exposure during Sleep Impairs Cardiometabolic Function.” Proceedings of the National Academy of Sciences of the United States of America 119, no. 12 (March 14, 2022). https://doi.org/10.1073/pnas.2113290119.
  • “Insulin under the influence of light – PubMed.” 21 Jun. 2020, https://pubmed.ncbi.nlm.nih.gov/32564344/.

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