Winter brings predictable changes — colder weather, shorter days, and a shift in routine. What many people don’t expect is how strongly it can affect mood, energy, appetite, and motivation.
Every year around late fall and early winter I start hearing similar concerns from patients:
“I’m sleeping more but still tired.”
“My motivation is gone.”
“I’m craving carbs constantly.”
“I feel flat or irritable and I don’t know why.”
Many people assume this is just stress, burnout, or the holidays.
Sometimes it isn’t.
Seasonal Affective Disorder (SAD) is a recurrent seasonal mood disorder that typically begins when daylight hours shorten. It affects sleep, appetite, concentration, and emotional regulation. This is not simply “winter blues.” It is a physiologic response involving circadian rhythm signaling, hormone regulation, and neurotransmitter production.
Light therapy and exercise are commonly discussed treatments, but one of the most overlooked contributors is nutrition status.
A Personal Perspective
I grew up in Alaska, where this condition is extremely common. During parts of winter, daylight can drop to only a few hours per day. People leave for work in the dark and come home in the dark for months.
My own brother struggled with it.
In our house we used “happy lights” at our desks every winter. It wasn’t alternative medicine — it was normal daily life. Families and schools used light exposure intentionally because mood changes were predictable every year.
What I didn’t understand growing up, but now see regularly in practice, is that light exposure was only one part of the physiology. Many people were also dealing with low vitamin D, disrupted sleep signaling, and altered serotonin regulation at the same time.
Can People in Texas Experience SAD?
Yes — and this surprises most of my Texas patients.
You do not need extreme northern darkness to experience seasonal mood changes. SAD exists on a spectrum. Full diagnostic SAD is more common at higher latitudes, but milder seasonal mood disturbance is very common even in southern climates.
Winter does not just mean less sunlight. It also means:
- Later sunrise
- Earlier sunset
- Less outdoor time
- More indoor work and screen exposure
- Decreased physical activity
- Lower vitamin D production
Even in Texas, daylight hours drop significantly between November and February. More importantly, most people are indoors during the hours when ultraviolet B (UVB) exposure is sufficient to produce vitamin D.
I routinely see vitamin D levels decline in patients every winter regardless of location.
Many patients who say, “I just don’t function well in winter,” are actually experiencing circadian rhythm disruption and neurotransmitter changes rather than a psychological problem.
What Is Happening Physiologically?
Seasonal Affective Disorder involves dysregulation of three major systems.
Circadian Rhythm
The brain’s internal clock relies on morning light exposure to regulate wakefulness, sleep timing, and hormone release. Reduced morning light shifts melatonin production later into the morning and suppresses daytime alertness.
Serotonin
Serotonin regulates mood, appetite, motivation, and emotional resilience. Reduced daylight decreases serotonin activity. This is strongly associated with fatigue, low mood, carbohydrate cravings, and brain fog.
Cortisol Rhythm
Cortisol should rise in the morning and decline at night. Winter light patterns often blunt this response, leading to low morning energy and poor sleep quality.
The Nutrition Connection
Neurotransmitters are not abstract brain chemicals. They are made from nutrients.
Serotonin synthesis requires:
- Tryptophan (amino acid from protein)
- Vitamin D
- Vitamin B6 and folate
- Magnesium
- Healthy gut microbiota
When one or more of these is insufficient, the brain has difficulty maintaining mood regulation.
Vitamin D and Seasonal Mood
Vitamin D functions more like a hormone than a vitamin. It participates directly in serotonin signaling and immune regulation.
Winter deficiency is extremely common. Even in sunny states, production drops because:
- Sun angle changes
- Skin exposure decreases
- People spend less time outdoors
For many individuals, supplementing vitamin D from fall through spring helps prevent deficiency and may support both immune function and mood stability during cold and viral season.
Dietary sources include fatty fish, egg yolks, and fortified dairy alternatives, but diet alone rarely maintains optimal levels during winter months.
Omega-3 Fatty Acids
Omega-3 fatty acids influence neuronal membrane function and inflammatory signaling in the brain. Low intake is associated with depressive symptoms and impaired concentration.
Sources include salmon, sardines, mackerel, flaxseed, chia seed, and walnuts.
Protein and Tryptophan
Winter carbohydrate cravings are often physiologic. Serotonin is produced from the amino acid tryptophan. Low protein intake reduces available substrate for neurotransmitter production.
Helpful protein sources include eggs, poultry, lentils, pumpkin seeds, and yogurt.
Balanced meals stabilize mood better than restrictive dieting during winter months.
The Gut–Brain Axis
Approximately 90% of serotonin signaling originates in the gut. Gut bacteria communicate with the brain through immune, inflammatory, and neural pathways.
Changes common in winter include reduced produce intake, less movement, more processed foods, and lower vitamin D levels. These can alter microbial diversity and contribute to mood symptoms.
Supporting gut health through fiber, fermented foods, and diverse plant intake often improves mood regulation.
Practical Strategies
Helpful interventions include:
Morning outdoor light exposure
Consistent sleep timing
Regular daily movement
Balanced meals with protein, fat, and fiber
Vitamin D assessment and supplementation when appropriate
Final Thoughts
Seasonal mood changes are not a character weakness or lack of motivation. They represent a biologic response to environmental change.
Light affects hormones.
Hormones affect neurotransmitters.
Neurotransmitters depend on nutrition.
Addressing both lifestyle and nutritional status provides a more complete approach than light therapy alone.
If you are noticing predictable winter fatigue, mood changes, or increased cravings each year, a personalized plan evaluating nutrition status, gut health, and lab markers may help.
You can schedule a free discovery call here
References
Lam, R. W., Levitan, R. D., Levitt, A. J., Enns, M. W., Morehouse, R., Michalak, E. E., & Tam, E. M. (2016). Clinical guidelines for the management of major depressive disorder with seasonal pattern. BMC Medicine, 14(1), 1–11.
Murray, G., Allen, N. B., & Trinder, J. (2003). Seasonality and circadian phase delay: Winter lowering of mood associated with shift toward eveningness. Journal of Affective Disorders, 76(1–3), 15–22.
Patrick, R. P., & Ames, B. N. (2015). Vitamin D and omega-3 fatty acids influence serotonin synthesis and action. FASEB Journal, 29(3), 761–768.
Liu, R. T., Walsh, R. F. L., & Sheehan, A. E. (2019). Prebiotics and probiotics for depression and anxiety: A systematic review. Neuroscience & Biobehavioral Reviews, 102, 13–23.
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