Updated 12/19/25
As motivation wanes with the season...
The leaves (and snowflakes) are falling, the wind has a chill, and the days are getting shorter. Autumn is here at last and winter doesn’t seem so far away for many across the nation – and although you might be looking forward to holidays and bonfires, you might have noticed your motivation waning as the day wears on.
You’re certainly not alone. There is something about the shorter days that messes with your motivation – and most of it has to do with variations in your biological clock. So much of how our body functions relies on environmental cues driven by the sun.1 All life revolves around it – figuratively and quite literally. In the summertime, we take it for granted, but with winter right around the corner, odds are you might be feeling a little blue – but it turns out your gut might be under the weather, too.
Winter Months and Seasonal Changes
Seasonal changes often bring shifts in how we feel and function. Growing up, you may have heard reminders to bundle up or stay dry during the wetter months. While temperature and weather don’t directly determine whether someone becomes ill, cooler seasons can influence factors that affect overall wellness.
For example, spending more time indoors may reduce sun exposure, which can impact Vitamin D levels. Seasonal changes can also influence your gut environment—an important part of supporting healthy immune function. These natural shifts highlight how your nutrition and lifestyle habits work together to help maintain overall well-being throughout the year.
Sunlight and Vitamin D
In spring and summer, when we walk outside and catch some rays, that sunlight causes all sorts of beneficial biochemical reactions in our skin. You might be familiar with UV rays – those things that sunblock is supposed to protect you from – but did you know that they also play an important role in keeping you healthy?
One of the few vitamins humans can create themselves is Vitamin D, essential for growth and immune function.3 We make this vitamin when certain UV rays hit our skin, converting cholesterol - used in our skin cell membranes to keep them fluid and mobile – into Vitamin D.4 Although you can ingest sources of Vitamin D in food or supplements (like fortified milk, yogurt, and fatty fish), the kind our skin produces is unique and is easier for us to process.5
Vitamin D is then used to promote a healthy immune system.*
The Immune System Organ
You might be familiar by now that upwards of 70% of your immune system is housed in your gut, making Vitamin D very interesting to some studies looking at gut health.* Recently, a study published in the scientific journal Frontiers noted that female participants in a small clinical study exposed to UVB light showed significant improvements in gut microbiome diversity.(9) The women showed improvements in gastrointestinal health.* This study showed a new perspective on how sunlight may impact our gut microbiome – something we never expected to see the light of day!
The exact way this impacts gut health, however, is proving to be a little more elusive - though the researchers in this study speculated it probably has to do with an improved immune system.* Their next step is to study how by examining the mechanisms that might be improving gut microbiome balance.*
Microbial Blues
If you’re feeling tired, lethargic, and – well – a little sad during the winter months, you aren’t alone.
Many scientists speculate lack of Vitamin D may be to blame since it contributes to our serotonin levels or our “happy” neurotransmitter.6 Serotonin works to regulate mood, sleep, appetite, digestion, and social behavior, making it a strong link in mental health studies. If you’re looking for another gut link – you don’t have to look hard: scientists estimate that 90% of the serotonin in our body is produced right in your gut.
In fact – most of that serotonin is made right from your gut microbes.7
If your gut is experiencing an imbalance, it could have a larger impact on your mood – something evidenced in studies exploring the gut-brain axis.8 It’s all connected!
Could it be that sunlight has a larger role in our gut health than previously thought? Researchers seem to think so.
The next time you’re feeling those seasonal blues, it’s not all in your head – in fact, it might even be in your gut!
What Can You Do?
The first step is remembering how much your body changes and adapts to the seasonal tides. We can’t bring back the sun, but you can certainly check in with your gut and see what’s happening inside- and Viome can help. If you’ve noticed a little more bloating or weight gain this season, it might not just be those second portions of pumpkin pie. Your gut might be experiencing changes based on the time of year and need a little extra attention – just like you do when it’s 5pm and dark outside, and the only thing you want is a heated blanket and your favorite Netflix show. We all need a little extra self-care this season!
Resources:
Honma K, Honma S, Kohsaka M, Fukuda N. (1992). Am J Physiol. 1992;262:R885-891.
van der Lans AA, Boon MR, Haks MC, et al. (2015). Physiol Rep. 2015;3.
Aranow C.(2011). J Investig Med. 2011;59:881-886.
Wacker M, Holick MF. (2013). Dermatoendocrinol. 2013;5:51-108.
Jungert A, Spinneker A, Nagel A, Neuhauser-Berthold M. (2014). Food Nutr Res. 2014;58.
Penckofer S, Kouba J, Byrn M, Estwing Ferrans C. (2010). Issues Ment Health Nurs. 2010;31:385-393.
Yano JM, Yu K, Donaldson GP, et al. (2015). Cell. 2015;161:264-276.
Clapp M, Aurora N, Herrera L, Bhatia M, Wilen E, Wakefield S.(2017). Clin Pract. 2017;7:987.
Bosman, E. S., Albert, A. Y., Lui, H., Dutz, J. P., & Vallance, B. A. (2019). Frontiers in Microbiology, 10, 477346. doi.org/10.3389/fmicb.2019.02410
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