We often think of Vitamin D in terms of bone strength or sun exposure—but its impact goes far deeper, especially when it comes to mental health.

Vitamin D behaves less like a traditional vitamin and more like a neuroactive steroid hormone. Its receptors are found throughout the brain, where it influences the production of mood-regulating neurotransmitters like serotonin and dopamine. In winter, when sunlight exposure drops, so can your Vitamin D levels—and for many, that means noticeable changes in mood, energy, and mental clarity.

The Brain on Vitamin D

Unlike nutrients that act locally, Vitamin D operates at a genetic level. Once activated in the body, it binds to receptors that regulate the expression of over 1,000 genes—including those involved in:

• Mood regulation (via serotonin synthesis)
• Immune system function
• Inflammation control
• Circadian rhythm synchronisation

Low levels of Vitamin D have been linked to increased risk of depression, anxiety, and even cognitive decline. Emerging research also shows that Vitamin D may help protect the brain against oxidative stress and inflammation, both of which are heightened in people with mood disorders.

Why We Struggle in Winter

During summer, we make Vitamin D naturally through skin exposure to UVB rays. But in winter:

• The sun sits lower in the sky, making UVB less effective
• We spend more time indoors
• We cover up more and use sunscreen—important for skin protection, but also a barrier to synthesis

By late winter, Vitamin D levels can fall to their lowest point of the year. In fact, research shows up to 36% of Australians are deficient in winter and early spring.

What to Watch For

Symptoms of low Vitamin D aren’t always obvious—but if you’re experiencing the following, it may be worth investigating:

• Low or flat mood
• Fatigue or sluggishness
• Difficulty concentrating (“brain fog”)
• Muscle aches or bone tenderness
• Frequent colds or infections

These overlap closely with symptoms of Seasonal Affective Disorder (SAD), a recognised form of depression triggered by low sunlight and disrupted circadian rhythms.

How to Take Action

If winter is affecting your mental clarity or energy, here’s how to support your health:

1. Get tested - a simple blood test is the most reliable way to check your Vitamin D status. At i-screen, our Vitamin D test makes it easy to take control of your health from home or at a local collection centre.
2. Use safe sunlight exposure - where possible, aim for 5–30 minutes of direct sunlight to the arms, legs, or face a few times a week. Mornings are best for regulating your circadian rhythm.
3. Consider supplementation - if your levels are low or you’re at risk of deficiency, a high-quality Vitamin D3 supplement may help. Always follow your healthcare provider’s guidance.
4. Support your body holistically - light therapy, exercise, quality sleep, and nutrient-dense foods also play a role in keeping your mood stable through seasonal transitions.

The Bottom Line

Vitamin D isn’t just about bones—it’s a powerful modulator of brain chemistry, mood, and immunity. If you're feeling the seasonal slump, it might be time to think of Vitamin D not as a vitamin, but as your winter hormone ally.

If you're worried you may have a vitamin D deficiency, then finding out is easy with a simple blood test - check out our Vitamin D Check for more details.