Can Microbes Squash the Winter Blues?

Written by Megan Falk: Experienced health and wellness journalist and editor. Megan is a graduate of Syracuse University’s S.I. Newhouse School of Public Communications, where she earned a bachelor’s degree in Magazine Journalism and a minor in Food Studies. She’s also a certified personal trainer through the American Council on Exercise.
Reviewed by Jennie O’Grady: Senior SciComms Specialist at Seed Health

Wake up to the tenth gloomy winter day in a row, and you might just become a real-life Debbie Downer, with a glum mood and a resting sad face to match. You’re not alone: 41% of Americans say their mood takes a turn for the worse in the winter, according to a recent American Psychiatric Association survey.¹

This seasonal shift makes sense. The combination of fewer daylight hours and chilly temperatures can make even the most upbeat people feel down in the dumps. But there’s another factor at play too: your gut microbiome. And unlike the solar zenith angle, this one falls largely within your control.

Let’s break down why your mood might dip during winter, cover when the winter blues become something more serious, and explore how your gut microbiome can be a crucial seasonal ally. We’ll wrap by sharing five tips to help you make your own sunshine—no matter the weather forecast. 

Darker Days, Darker Moods

First, a little geoscience refresher: Thanks to the Earth’s tilt, there are fewer daylight hours throughout the winter months. Depending on your location, the difference in sunlight can be drastic. This year, Juneau, Alaska will see the sun for just 6 hours and 23 minutes on the winter solstice, while the city basked in over 18 hours of daylight on the summer solstice.²

Each additional hour spent in outdoor sunlight during the day has been linked with lower odds of major depressive disorder, antidepressant use, and poor mood, as well as greater happiness, according to a 2022 study of more than 400,000 people.³

On the other hand, darker days can have a profoundly negative impact on your mind—and cause ripple effects throughout your body. Let’s dig into why that is: 

• Sunlight exposure is one of the main ways your body synthesizes vitamin D. The aptly nicknamed “sunshine vitamin” may influence mental and emotional health through a sequence of intricate biological processes.⁴ First, vitamin D activates an amino acid called tryptophan. Then, tryptophan is synthesized into serotonin—one of the hormones that helps you feel mentally balanced and calm. According to one small study, sunlight also keeps serotonin levels up by decreasing the activity of a protein called serotonin transporter (SERT). SERT’s job is to collect and recycle serotonin after it’s been released. When its activity is reduced, more serotonin stays available for your brain to use. In some people, shorter days and longer nights (i.e. less sunlight) lead to higher SERT activity, which reduces the amount of serotonin available to the brain.⁵ 
• Exposure to sunlight regulates your body’s circadian rhythm—and, in turn, the natural ebbs and flows of certain hormones.⁶ Disrupted circadian processes may contribute to mood and sleep disorders during winter.⁷
• With the sun setting earlier, your body may also release more melatonin into your bloodstream.⁸ The problem: The happy hormone serotonin is used to create melatonin. During longer periods of darkness, your body may deplete your serotonin supply to keep the melatonin pumping—and that drain on serotonin can affect mental health. 
• Finally, winter conditions can make it more challenging to live a healthy lifestyle. Having less access to daylight has been linked with reduced physical activity, which might negatively impact mood and quality of life.^9,10 If you live in an area that gets cold during winter, exercising (or simply leaving the house) becomes even more challenging. 

When Sad Becomes SAD

For some people, the downcast disposition that persists through the colder, gloomy months isn’t just a case of the “winter blues.” It’s veered into seasonal affective disorder, aka SAD.

SAD is a type of clinical depression that impacts daily life. It occurs during specific seasons and typically lasts for about 40% of the year, beginning in late fall or early winter and resolving in spring or summer. (It’s possible to experience SAD in the summer, too, though it’s not as common.)^11,12

Those who suffer from SAD exhibit depressive symptoms, including a loss of interest or pleasure in activities previously enjoyed; feelings of hopelessness, guilt, or worthlessness; and persistent sadness or depressed mood. They may also deal with sleep disturbances, food cravings, or the desire to withdraw socially during certain seasons.¹² 

So what’s the line between the diagnosable condition and simply feeling blue? To be diagnosed with SAD, folks need to have symptoms during the same season for at least two consecutive years, and these symptoms must interfere significantly with day-to-day functioning.¹³

The symptoms of the winter blues may be similar to those of SAD, but they are generally milder, temporary, and less impactful to daily life. 

Winter blues can generally be relieved with the help of simple lifestyle changes, such as spending more time outside and working more movement into your day. SAD, on the other hand, isn’t as quick of a fix. If you show any of the symptoms, that’s your sign to chat with a healthcare provider.

How Your Gut Microbes Influence Your Winter Mood

Clearly, your outside environment (sunlight, temperature, etc.) influences your mood each season. But your internal ecosystem does too. The composition of microbes within your gut can impact your mental health in winter for the following (fascinating 🤓) reasons:

1. Vitamin D is a major player in gut health. 

Aside from its role in regulating serotonin production, vitamin D from UVB light has been shown to support the integrity of the gut barrier and increase microbial diversity (a key indicator of gut health), as shown in a small pilot study on healthy young females with relatively pale skin.^14,15 (It’s worth noting that we could use more research in a more diverse group of participants, though.) 

Without enough vitamin D, the gut microbiome’s composition may take a turn for the worse. A lack of vitamin D can also negatively affect the gut-brain axis—the telephone line between the gastrointestinal tract and central nervous system that influences mood and behavior.¹⁶ 

Ultimately, these microbial shifts could play a role in the development or exacerbation of the winter blues or SAD symptoms. Exhibit A: Those with depression may show altered gut microbiota profiles, with fewer “good” bacteria and more that are potentially harmful.¹⁷ 

2. Gut microbes help keep your dopamine levels on point.

Changes to the gut microbiome that influence the production of “feel good” neurotransmitters like dopamine can also impact mood. 

In a strong, resilient gut microbiome, gut microbes and their metabolites play a significant role in dopamine production and regulation. Others serve as bodyguards for “dopaminergic neurons,” protecting the neurons from damage.¹⁸ 

Meanwhile, there are dopamine villains in the gut, too. Some microbes can trigger the release of endotoxins—molecules that set off an immune response and cause inflammation throughout the body. That process can damage those dopaminergic neurons and slash dopamine production and release.¹⁸

In other words, balancing beneficial and harmful gut microbes is essential for keeping your dopamine levels *just right*.

3. Your gut is responsible for the vast majority of your serotonin production.

As mentioned, serotonin is used to create melatonin and, as a result, supports healthy sleep-wake cycles. The kicker: Roughly 90% of your body’s serotonin is produced in your gut.¹⁹ And if your microbiome isn’t in a well-balanced, diverse state, your serotonin levels may be impacted. Cue the tired, crying emojis.

4. Your GI tract also pumps out sleep hormones. 

Along with the pineal gland—a tiny endocrine gland in the brain—the GI tract also produces melatonin. Your resident microbes assist in the process of converting tryptophan to serotonin and finally to melatonin, and melatonin itself supports the proper composition and dynamic of the gut microbiota.^20,21 

This means a change in the gut microbiome could lead to a change in melatonin production, potentially affecting mood, sleepiness, and other factors that fall under the hormone’s domain.

5 Ways to Boost Your Mood—and Microbiome—This Season

Winter is coming. Add these mood- and gut-enhancing tips to your toolkit, and you may find yourself feeling brighter even during the darkest days of the season. 

1. Pencil in sunshine (or sunlamp) time. Treating yourself to light during the day helps maintain adequate melatonin and serotonin production, and studies have shown that it can improve short- and long-term mood.^22,23 Head outdoors during the daylight hours, keep your blinds open, and re-arrange your home or office space so you’re getting sunlight as much as possible throughout the day. Using a sun lamp or light therapy box can also work in a pinch. Research shows that sitting in front of one for 20–60 minutes per day, typically first thing in the morning, can ease symptoms of SAD in as little as 2–4 weeks.^24,25,26,27
2. Get your body moving. Elle Woods was right: Exercise gives you endorphins, and endorphins do make you happy. Research shows that exercise may be as effective for reducing depression as cognitive behavioral therapy or medications, and combining exercise with antidepressants tends to improve symptoms more than medication alone.^28,29,30 As a whole, regular exercise can also increase the number of beneficial microbial species in your gut and enhance microflora diversity.³¹ Take a bundled-up walk or jog in your neighborhood (or on a treadmill), flow through a yoga video, or lift weights to reap the benefits for your gut-brain axis.
3. Build opportunities for connection into your day. As cozy as your couch may be, hibernating at home throughout the winter may worsen mood, and lacking social support is a risk factor for depression.^32,33 Join a book or run club, slot regular coffee chats with friends on your calendar, or volunteer in your community to get your daily dose of social interaction. 
4. Focus on fiber, fermented foods, fatty acids, and vitamin D. Eating a gut-friendly diet can promote both physical and mental health in wintertime (and all year round). Everyone’s nutrition needs are different, but for the most part, fiber-rich fruits, vegetables, legumes, nuts and seeds; fermented options; and omega-3 fatty acids tend to be best for ​​nurturing the microbiome and mental health by extension.^34,35 With fewer opportunities to douse yourself in sunshine in winter, you can also prioritize foods high in vitamin D (oily fish and eggs, to name a few), which may help support the production of mood-regulating serotonin. A daily vitamin D supplement may also support mood, though current research findings are mixed.^36,37,38
5. Book an appointment with your healthcare provider if you think you might have SAD. If your seasonal symptoms are significant, long-lasting, and are impacting your daily life, chat with a healthcare provider. They can help you determine if you’re experiencing SAD and develop a personalized treatment program. They may recommend common methods such as talk therapy (particularly Cognitive Behavioral Therapy), antidepressant medications, or a combination of multiple options.³⁹

The Key Insight

A lack of vitamin D- and serotonin-producing sunshine, plus an influx of melatonin, can dampen your mood throughout winter. Tending to your gut health may help you get through the season feeling more like yourself. 

Winter isn’t the only time that it pays to know about the gut-brain axis. Read the latest research on this superhighway between the gut and the brain—and its many implications for your health.

If you feel your depression is severe or if you are experiencing suicidal thoughts, consult a physician immediately or seek help at the closest emergency room. The 988 Suicide & Crisis Lifeline provides 24/7, free, and confidential support for people in distress as well as prevention and crisis resources for you or your loved ones. Call or text 988 or online chat at 988lifeline.org.