With Halloween behind us, many people are already looking forward to the festive season as a way to cope with the dreary winter months.

The endless rain, freezing temperatures, and long, dark days can take a toll on both mood and energy levels. There’s a reason we feel the “winter blues”—and it all comes down to the sun, or rather, the lack of it.

Why Sunshine Matters

Sunlight is essential for our health because it helps our bodies produce vitamin D, a hormone vital for maintaining overall well-being. According to the NHS, vitamin D regulates calcium and phosphate levels, which are needed for strong bones, healthy teeth, and proper muscle function. It also supports good sleep and emotional balance.

But as daylight hours shrink and skies grow darker, our bodies produce far less vitamin D. That’s why many people turn to supplements during the colder months to make up for what they’re missing.

A Doctor’s Warning About Deficiency

Health experts have now issued a reminder for anyone taking vitamin D supplements: be aware of the warning signs of deficiency.

While there are several potential causes—such as age, diet, or certain medical conditions—the main culprit during winter is the reduced exposure to sunlight.

Nutritional therapist Eve Kalinik explained that one of the first signs of deficiency is a sudden “susceptibility to infections and viruses,” since vitamin D plays a key role in immune defense. She also noted that symptoms can include “bone pain or muscle weakness (as vitamin D helps regulate calcium levels in the body), as well as low mood and fatigue, which are often linked to Seasonal Affective Disorder (SAD).”

Overlapping Symptoms and Hidden Signs

Fatigue is often the earliest symptom, but it can easily be overlooked during the chaotic winter months. Factors like disrupted sleep from the time change, stress, or hormonal shifts can mask vitamin D deficiency. This tiredness can then spiral into other problems, such as frequent colds, poor mood, and weight gain.

If left unchecked, low vitamin D levels can contribute to bone loss, leading to conditions like osteoporosis or osteomalacia—both of which increase the risk of fractures later in life. In severe cases, deficiency may even cause seizures.

Testing for Vitamin D Levels

If you suspect you might be deficient, an at-home blood test can help. These kits are widely available and provide a clear picture of your vitamin D status. The results can also show just how low your levels are, allowing you to adjust your supplements or diet accordingly.

As winter sets in and sunlight grows scarce, keeping an eye on your vitamin D levels can make a major difference to your health, mood, and energy throughout the colder months.