The Scientific Reasons You Feel More Tired During Winter (And How To Combat It)

Your body is hardwired to be more fatigued during winter. Here's what you need to do to keep your energy levels high.

When the winter months hit, many of us feel the need to hibernate.

Suddenly, getting out of bed in the morning is a Herculean task. Mid-day fatigue rises to another level. Going to the gym—a task that was easy during the summer and fall—now feels nearly impossible.

Is this just your imagination? Probably not. Odds are it's winter-related fatigue. There are scientific reasons people feel more tired during winter than they do during other seasons. The good news is you can take steps to fight the fatigue and stay energetic, even during the darkest days of winter.

Less Sun Makes Us More Sleepy

Sleepy Man with Alarm Clock

Stepping outside on a sunny day is one of life's simple pleasures. It's also something we don't get the chance to do very often during winter.

For one, the days get shorter during winter. This is especially true for those living in northern cities. For example, take a city like Cleveland, Ohio. On June 21 (the summer solstice), the sun was in the sky for 15 hours, 10 minutes and 21 seconds. On December 21st (the winter solstice), the sun will be in the sky for 9 hours, 10 minutes and 11 seconds. That's over six hours of less daylight.

While everyone enjoys a nice day, sunlight is closely tied to human biology. Melatonin is a hormone produced by the pineal gland inside the brain. Melatonin regulates sleep and wakefulness. When we're in the dark, our bodies produces more melatonin. Winter is a dark time, so our bodies produce more melatonin in response. This leads to excessive feelings of fatigue and tiredness. According to the Mayo Clinic, "the change in season can disrupt the balance of the body's level of melatonin, which plays a role in sleep patterns and mood."

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Sunlight is also our major source of vitamin D. Human skin creates significant amounts of vitamin D when it's exposed to sunlight. Those who live in northern latitudes tend to have lower vitamin D levels, especially during the winter months. When the weather is cold and the days are short, there are fewer opportunities to get outside.

Vitamin D deficiency is rampant in our society, and not just in colder climates. A recent study found that more than one-third of the student-athlete population at the University of Southern California had low levels of vitamin D.

Vitamin D has a huge impact on how we feel. It plays a role in bone health, cell growth, blood pressure, immune function and reduction of inflammation. It also plays an important role in performance and recovery. "Vitamin D is so important for performance. We used to think it only impacted bone health, but more and more studies have shown that it acts like a hormone and actually has a role in muscle function. It's very important for athletes," says Dr. Maren Fragala, Director of Athlete Health and Human Performance for Quest Diagnostics.

Vitamin D has been found to impact performance in a variety of ways. A recent study conducted by Precision Nutrition found that when athletes entered a workout with "optimal serum vitamin D concentrations," they went on to recover faster and more efficiently. "More pre-exercise vitamin D meant less post-exercise muscle weakness and better recovery through the entire recovery process. Less pre-exercise vitamin D meant more weakness and worse recovery," the study's authors wrote.

A different study, published in the journal PLOS One, found that vitamin D levels were strongly correlated with the performance of professional soccer players. "Findings suggest that vitamin D levels are associated with the ergonometric evaluation of muscle strength . . . sprinting capacity and VO2 max in professional soccer players," the authors wrote. Low levels of vitamin D increase fatigue and make recovery take longer. No wonder getting to the gym on a regular basis feels so difficult during winter!

RELATED: A Growing Number of Elite Athletes Are Deficient In This Incredibly Important Vitamin

Fighting Winter Fatigue

Running on Road

There are a number of simple steps you can take to boost your energy during the cold, dreary months.

Getting enough vitamin D is a great start. How much vitamin D do you actually need? The Institute of Medicine recommends 600 International Units per day for most adults, but many sports dietitians recommend higher amounts for athletes. A recent article from The Wall Street Journal states that "some sports dietitians encourage athletes to get 1,000 to 2,000 IU [of vitamin D] daily."

Sunlight is obviously the best option, but getting enough vitamin D from sunlight is especially tough during winter. According to the National Institute of Health, skin exposed to sunlight through a window does not produce vitamin D. That means you have to actually go outside to collect vitamin D from sunlight. This might sound tough during winter, given that the days are short and the weather is nasty. However, your skin does receive vitamin D even on cloudy days—just not as much as it would on a day with blue skies. If you're able to go for a walk or jog outside a few times a week, that should help.

It's also possible to get vitamin D from your diet, but relatively few foods contain a significant amount. Examples of foods high in vitamin D include egg yolks, fatty fish and fortified products like cereals and milk.

Since adequate amounts of vitamin D can be hard to come by, many take it in the form of a supplement. If you go that route, select a vitamin D3 supplement, since that form most closely approximates the vitamin D naturally produced by the body.

Besides vitamin D, regular exercise is a surefire way to boost energy levels. This might sound counterintuitive, but research backs it up. A study from the University of Georgia found that sedentary adults who engaged in as little as 20 minutes of low-to-moderate aerobic exercise three days a week for six weeks experienced a significant uptick in their overall energy levels. I get that you might not want to drive to the gym in a blizzard and knock out a 60-minute workout. But doing something always beats doing nothing. Winter is a perfect time to use convenient at-home workouts such as yoga or TRX-based training.

RELATED: The 10 Best Yoga Poses For Athletes

Diet is another important factor. It often feels easier to eat fruits and vegetables during the summer months, but winter is when your body really craves their nutrients. Keep your plates colorful and include a variety of produce to help keep your energy levels high.

Light therapy lamps (or boxes) are another option. "A light therapy box mimics outdoor light. Researchers believe this type of light causes a chemical change in the brain that lifts your mood and eases other symptoms of seasonal affective disorder," writes the Mayo Clinic. Amazon offers a wide variety of such products for under $100. If you're interested in learning more, the Mayo Clinic has an informative page on the topic.

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