If winter leaves you feeling drained, moody, or glued to the snack drawer, you’re not alone. And science backs it up.
Research shows that both physical and mental health shift with the seasons, with winter ranking as the worst for physical health, and spring and fall topping the list for poor mental health.
That seasonal rollercoaster doesn’t just change your calendar. It changes your biology.
One of the clearest examples is Seasonal Affective Disorder (SAD), a form of depression that mostly shows up in the fall and winter.
SAD affects about 5% of U.S. adults, with women making up nearly 80% of cases.
Symptoms can feel eerily familiar to daily life: fatigue, mood swings, carb cravings, and sleep struggles. SAD can mirror every day feelings.
SAD is also more common in places farther from the equator, where winter daylight hours are short and your body’s natural rhythm gets thrown off.
Here’s the thing: while some people breeze through December like it’s just another season, others feel anxious, inflamed, or completely wiped out before January begins.
The answer lies in how our bodies are wired to respond to seasonal cues.
For many, those cues mean lower energy, shifts in mood, and recurring health struggles. Seasonal fatigue reflects deeper changes in hormones, metabolism, and inflammation.
With the right testing tools, functional medicine providers can tailor support to the season, helping patients stay resilient even when winter works against them.
Ahead, we’ll break down the science of winter fatigue and explore how functional medicine testing can help practitioners identify what’s really going on beneath the seasonal slump.
Want deeper insight into your patients’ seasonal health shifts? Explore Access Medical Labs Functional Health Testing to see how comprehensive panels can reveal what’s driving fatigue, mood changes, and immune stress.
Let’s start with the obvious: shorter days mean less sunlight.
That reduced light exposure messes with your circadian rhythm, the body’s internal clock that controls sleep, energy, and mood.
When melatonin rises too early and serotonin dips too low, you’re left with sluggish mornings and nights full of tossing and turning.
And there’s proof that this isn’t just an in-person experience. A long-running survey tracked how often U.S. adults reported feeling physically or mentally unwell each month.
People are objectively sickest in winter, with January showing about 0.6 more “unhealthy days” per person than summer, and their mental health dips the worst in spring and fall.
They don't call vitamin D the "sunshine vitamin" for nothing.
With less sunlight exposure in winter, vitamin D levels plummet, and with that comes fatigue, mood instability, and weaker immune defenses.
Nearly 40% of U.S. adults are vitamin D deficient, and winter only widens that gap. For practitioners, checking Vitamin D 25(OH) levels can reveal whether fatigue and low mood are less about motivation and more about biology.
In one of our Access Live webinars, attendees heard from Dr. Staci Blume, a leading chiropractor and functional medicine clinician. As the co-owner of Well Infused, a holistic health spa specializing in bioidentical hormone therapy and IV nutrition, Dr. Blum brings deep expertise and a trusted voice in integrative health.
Dr. Blume recommends monitoring vitamin D levels closely and maintaining levels above 70 ng/mL, noting that this supports not only immune modulation but also cardiovascular health and mitochondrial energy production.
But vitamin D isn’t the only marker worth watching. Functional medicine testing often reveals that thyroid imbalances, especially subclinical hypothyroidism, can mimic “winter tiredness” and amplify seasonal fatigue.
Incorporating thyroid panels alongside vitamin D testing helps practitioners build a fuller picture of hormone health and energy regulation.
Ever wonder why winter makes you crave pasta and bread as if it were the only food group that mattered?
Blame serotonin.
There’s science behind the cravings. Reduced sunlight lowers serotonin, the neurotransmitter that regulates mood and appetite, while shorter days trigger melatonin spikes that leave you groggy. That combo can tank mood and increase carb cravings.
Symptoms overlap with classic depression, but they flare specifically in the fall and winter when daylight hours shrink.
Research also finds that light therapy improves symptoms in 60–80% of patients, highlighting just how closely mood regulation is tied to seasonal light exposure.
Alongside standard light-box treatments, photobiomodulation (PBM), a therapy using red and near-infrared light, is showing promise in supporting seasonal mood shifts.
PBM targets cellular energy by stimulating mitochondrial enzymes and reducing inflammation, potentially improving brain function and resilience during dark months.
While light therapy helps reset your internal clock, PBM works deeper, powering your cells to rebalance mood chemistry from within.
Cortisol is often called the body's "get up and go" hormone.
Under normal conditions, it peaks in the early morning, helping you wake up alert, and gradually tapers through the day so you can wind down at night.
But winter's reduced daylight often flattens or delays this rhythm.
That's why you wake up sluggish, then feel wired and restless long after bedtime, a hallmark of seasonal fatigue and circadian disruption.
Research backs this up. Studies show that circadian misalignment increases the risk of insomnia, metabolic dysfunction, and depression.
Light exposure is key: morning sunlight helps reset cortisol and melatonin cycles, while dim light exposure at night worsens rhythm disruption.
Newer findings go deeper. A 2022 study found that seasonal shifts in daylight directly influence HPA axis activity, altering cortisol secretion and contributing to winter fatigue and mood instability.
In this video, Dr. Mitch Ghen explains how stress disrupts the HPA axis and cortisol balance, contributing to metabolic, cardiovascular, and immune dysfunction. A must-watch for providers looking at the science of stress and seasonal fatigue.
Another 2023 review highlighted how irregular cortisol and melatonin rhythms are increasingly tied to higher rates of sleep disorders, immune dysregulation, and mood disturbances in darker months.
Winter has a reputation for being the season of colds and flu. And science shows why.
Beyond crowded indoor spaces, the immune system itself follows a seasonal rhythm.
Studies demonstrate that immune defenses are weaker in winter, with reduced activity in key white blood cells and shifts in inflammatory markers like C-reactive protein (CRP) that increase vulnerability to infection.
Nutrient status plays a major role.
Low levels of vitamin D and zinc, both common in colder months, are strongly linked to diminished antiviral defenses.
Research confirms that vitamin D deficiency contributes to higher rates of respiratory tract infections during winter. Zinc depletion, meanwhile, reduces T-cell function, leaving the body less prepared to fight viral threats.
In this episode of True Health News, Dr. Mitch Ghen explains why patients may remain vitamin D deficient despite regular supplementation.
He highlights factors like malabsorption, medication interactions, obesity, genetics, and co-factor imbalances, all of which can compromise absorption and immune resilience. For clinicians, it’s a practical guide to troubleshooting stubborn vitamin D deficiency and improving patient outcomes.
Sleep disruption compounds the problem. Shorter days, circadian misalignment, and stress can blunt the immune response further.
One study highlights how these combined winter effects create the perfect conditions for viral spread and make recovery slower, contributing to lingering post-viral fatigue in some patients.
This seasonal immune stress explains why many patients report repeat infections, slower healing, and chronic fatigue that persists well beyond the initial illness.
For practitioners, monitoring immune markers and nutrient status during winter offers key insights into patient resilience and infection susceptibility.
As the temperature drops, so do healthy eating habits.
Comfort foods, often higher in sugar, refined carbs, and saturated fats, tend to replace lighter, nutrient-dense summer meals.
Combined with less outdoor activity and reduced sunlight, these dietary and lifestyle shifts can drive low-grade systemic inflammation, contributing to weight gain, joint discomfort, autoimmune flares, and metabolic stress.
The gut microbiome also changes with the seasons. Research shows that gut microbial diversity tends to decline in winter, reducing the production of short-chain fatty acids (SCFAs) that normally help regulate inflammation.
Lower microbial diversity has been associated with greater susceptibility to inflammatory conditions and even mood disorders.
Inflammatory markers reflect this seasonal pattern. Studies have found that CRP levels and monocyte counts run higher in winter months, highlighting a measurable seasonal shift in inflammatory tone. These changes not only raise infection risk but can also worsen fatigue and mood swings, making patients feel depleted well beyond diet alone.
The takeaway: winter isn’t just harder on willpower, it’s harder on biology.
Here’s the bottom line: winter fatigue isn’t just “all in your head.”
It’s biology responding to seasonal stress.
Functional medicine practitioners can move beyond guesswork by integrating seasonal testing, pinpointing whether fatigue stems from cortisol rhythm changes, vitamin D deficiency, neurotransmitter imbalance, or systemic inflammation.
Access Medical Labs offers panels that align with these needs, including cortisol rhythm testing, vitamin D status, thyroid health, and immune assessments. For practitioners, this means turning vague symptoms like “I’m so tired every winter” into targeted, personalized protocols that evolve with the seasons.
Winter symptoms don’t have to be a mystery. Access Medical Labs offers functional health testing that helps uncover the root causes of fatigue, mood shifts, and immune stress.
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Disclaimer: Content on the Access Medical Labs blog is for informational purposes only and reflects the views of individual contributors, not necessarily those of Access Medical Labs. We do not endorse specific treatments, products, or protocols. This content is not a substitute for professional medical advice. Always consult a qualified healthcare provider regarding any medical concerns.