Introduction
A few weeks ago, a patient with a stable diagnosis of depression visited my office. Despite months of stability, they were experiencing a change in mood: feeling more tired and irritable with a mental fog they couldn’t shake. This shift coincided with shorter days and diminished sunlight, a common trigger for Seasonal Affective Disorder (SAD).
The Impact of Light on Biological Processes
For millennia, life on Earth has been regulated by light. Even before clocks and calendars existed, our bodies knew when to sleep and wake due to the alternation between day and night. This internal clock, governed by a small brain structure called the suprachiasmatic nucleus, responds to light entering the eyes and sets the rhythm for biological processes such as temperature, digestion, sleep, energy levels, immunity, and mood.
As days shorten and solar radiation decreases, some animals hibernate, reduce their metabolism, or stop reproducing. This isn’t laziness but a biological strategy for surviving winter.
Though modern humans no longer hibernate, we remain strongly tied to seasons. Decreased natural light alters circadian rhythms, affecting the secretion of melatonin—the hormone regulating sleep. Additionally, serotonin levels drop, linked to well-being, and cortisol production changes, affecting stress regulation. Essentially, our bodies enter ‘winter mode’.
The Brain’s Response to Less Light
Insufficient sunlight exposure disrupts circadian rhythms and affects neurotransmitter regulation, such as serotonin and dopamine. This explains why many people exhibit symptoms of melancholy, fatigue, irritability, or difficulty concentrating during these months.
Known as seasonal affective disorder (SAD), it’s estimated to affect 2-10% of the population, depending on latitude. In northern European countries where sunlight can disappear for weeks, prevalence may double compared to summer. Even in places like Mexico City, where winter isn’t harsh, many experience mood dips, sleep disturbances, or appetite changes.
Intestinal Effects
Intriguingly, light also influences the gut microbiota—a network of bacteria involved in digestion, immunity, and mental health.
Canadian researchers found that exposure to UVB light increased microbiota diversity in individuals with vitamin D deficiency. Other studies identified seasonal variations in composition: more Actinobacteria in summer, more Firmicutes in winter. These fluctuations can impact intestinal serotonin production, responsible for up to 90% of the body’s serotonin.
Thus, less light can mean less vitamin D, lower bacterial diversity, and increased emotional vulnerability.
Embracing the Indoor Winter
Historically, agricultural cultures understood that seasons transform not just the land but also the spirit. Autumn marked harvest time, introspection, and igniting inner warmth. Today, however, we inhabit an artificial spring year-round: electric lights, screens on, constant work, and little respect for the body’s natural rhythms.
Yet, our bodies defend themselves. They crave rest, warmth, and slowness while we punish them with excessive caffeine, late-night screen time, constant heating, and closed windows.
The good news is that there’s much we can do to prevent SAD and maintain mood during this season:
- Get natural light. Go outside, even if it’s cloudy. Just 20 minutes of morning exposure stabilizes your internal clock. Fototherapy lamps (at least 10,000 lux) can also treat SAD.
- Regulate your circadian rhythm. Try to go to bed and wake up at the same time daily. Avoid blue light from screens at least two hours before bedtime.
- Eat seasonally. Nature provides what we need: citrus fruits rich in vitamin C, root vegetables strengthening vital energy, and legumes providing tryptophan, a serotonin precursor. Fermented foods like kéfir and kimchi also help balance gut bacteria.
- Move and breathe. Exercise releases endorphins countering winter lethargy. Simple strategies like mindful breathing, grounding (walking barefoot on earth or grass), and meditation reconnect body and mind.
- Reinforce as needed. Check your vitamin D levels; if low, consider supplementation during winter with medical guidance. Some people also benefit from omega-3, magnesium, and seasonal probiotics.
- Inner light. Create well-being rituals like lighting a candle at dusk, reading before bed, preparing warm infusions, or simply expressing gratitude for the day. Neuroscience shows these conscious gestures reduce amygdala activity and improve emotional regulation.
A Doctor’s Perspective
When my patient finished describing their low energy levels, I told them something they didn’t expect:
“Your body is doing the right thing. It’s reacting to winter.”
I advised them to go outside each morning, eat fruits and roots, walk under the sun, and dim home lights in the evening.
Two weeks later, they returned with more energy and less mental fog.
Perhaps the biggest lesson of this season isn’t fighting cold and darkness but learning to inhabit them.
I’d love to hear your questions or experiences related to this topic. Let’s keep the conversation going; you can email me at [email protected] or reach out on Instagram at @dra.carmenamezcua.
