Do you ever notice that you feel differently in the summer and winter? That you crave different foods, have different sleeping patterns, or experience different moods? Well, we are circadian beings, which means our ability to survive and thrive as a species depends on our connection to the rhythms of nature.
We now know from the last remaining hunter-gatherer tribe, the Hadza, that our gut microbes are designed to change from one season to the next.1 We know that microbes in the soil change seasonally and they have interdependent relationships with plants and microbes in our guts.2,3
Furthermore, we also know that carbohydrate-digesting microbes (Bacteroidetes) flourish in the summer, when more complex carbohydrates are harvested, and fat-eating microbes (Actinobacteria) flourish in the winter, when a higher-fat, higher-protein diet is available. Clearly, science points us in the direction of seasonal eating as a primary tool to stay in circadian rhythm.4
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Emerging studies find that digestive efficiency, in particular digestive enzymes, changes with the seasons.5 Both starch-digesting amylase and the digestion-promoting parasympathetic nervous system increase as temperatures cool in the fall and winter.6
Of course, due to the fall harvest of starches and grains, it makes sense that we produce more amylase. Ayurveda says that digestive strength is stronger during winter months, which would be needed to break down heavy and dense foods: think nuts, tubers, and meats.
It also makes sense that the rest-and-digest parasympathetic nervous system increases during colder months, when we need warmth and a stronger digestive fire.6
Another difference is that receptors for neurotransmitters like serotonin, dopamine, and brain-derived neurotrophic factors (BDNF) are more receptive in the light-filled summer months and much less receptive in the darker winter months. This explains why we see more mood-related concerns and depression during winter.7
Studies in both animals and humans have both shown increased microbial diversity in the gut during winter and spring, with much less in summer and fall.7 Opportunistic harmful bacteria also increase in winter and spring, so we need our immune strength most. Greater microbial diversity has been associated with greater health and immunity, which is much needed during cold winter and wet spring.4,6,7
Melatonin levels surge in winter, when daylight is less. This acts as a natural birth control agent for mammals. Conceiving in winter would render the baby premature to handle the cold the next winter. Melatonin is also the body’s most powerful immune-boosting, detox, and repair molecule, all of which we need more of during darker winter months.8,9,10
How Delicate Are Your Circadian Rhythms?
In a recent study, gut bacteria in two groups of mice were measured during normal exposure to light-dark cycles for two days.2 One group of mice was healthy and the other had their circadian clock disabled to mimic jet lag.
The group that had the disabled circadian clock saw disturbed function of gut bacteria. Most notable was that normal feeding signals governed by gut bacteria were masked, so those mice ate incessantly during the two-day trial.
In fact, loss of circadian rhythms altered their microbiome so it became very similar to that of mice and humans with high blood sugar and obesity.10
The same study also compared the microbiomes of a small group of humans. One group flew cross-continentally and the other stayed in the same time zone. The group who flew overseas had microbiome changes similar to mice with disabled circadian clocks.
Jet-lagged humans had microbiological changes that put them at a higher risk for obesity and type 2 diabetes.10
According to Ayurveda, living in sync with natural circadian rhythms is key to health and longevity. So don’t be surprised if you feel changes with travel or the seasons, and take care of yourself as best you can!