Every September, I start to feel it — that subtle shift in energy, the later mornings, the earlier evenings. And honestly? My sleep starts to feel a little... off. When I began researching why, I kept landing on the same culprit: vitamin D. About half of all Irish people have vitamin D levels below the optimal threshold — and most of us have no idea that it could be quietly wrecking our sleep.
Does vitamin D actually affect sleep?
Yes — and the research on this is a lot stronger than most people realise. A growing body of evidence links low vitamin D levels to shorter sleep duration, poor sleep quality, and a significantly higher risk of sleep disorders. A 2025 study published in the American Journal of Physiology found that people with lower circulating vitamin D experienced far greater sleep variability night to night — meaning less predictable, less restorative sleep. Across multiple studies, vitamin D deficiency has been associated with roughly a 50% higher risk of developing sleep disorders. That's not a small number.
What's striking is how common deficiency is in Ireland. Our northerly latitude means we simply don't get enough sunlight — especially from October through March — for our skin to synthesise adequate vitamin D. Up to 24% of Irish adults are deficient during winter months, and approximately half the population has levels below the recommended threshold of 50 nmol/l year-round. If you're lying awake wondering why you can't sleep, your vitamin D levels may be worth a look.
How does vitamin D actually help you sleep?
This is where it gets interesting — and it goes much deeper than most people expect. Vitamin D works through a receptor called the VDR (Vitamin D Receptor), and these receptors are found throughout the brain, including in the hypothalamus and the raphe nuclei — the very regions that regulate your sleep-wake cycle.
Here's the mechanism that surprised me most: vitamin D directly regulates an enzyme called AANAT, which is the rate-limiting step in melatonin production. In other words, without adequate vitamin D, your body may struggle to produce enough melatonin — the hormone that signals it's time to sleep. On top of that, vitamin D influences the expression of core circadian rhythm genes, including Clock, Bmal1, and Per2. These are the genes that set your body's internal clock. When they're not functioning properly, your sleep timing and quality can both suffer.
Vitamin D also helps reduce inflammatory cytokines that can disrupt sleep architecture — so if you're waking frequently through the night, low-grade inflammation driven by vitamin D deficiency could be a contributing factor.
What type of vitamin D is best for sleep — and do I need K2 with it?
The form that matters is D3 (cholecalciferol) — not D2, which is less bioavailable and less effective at raising blood levels. D3 is the same form your skin makes from sunlight, which is why it's the more relevant form for most people.
The reason I pair D3 with K2 in Harmony is simple: vitamin D increases calcium absorption, and K2 is what directs that calcium to your bones and teeth — rather than your arteries. Together, D3 and K2 work synergistically. I also include magnesium bisglycinate in Harmony because magnesium is required to activate vitamin D in the body, and many Irish people are low in both. Addressing them together gives you a much more complete picture.
I designed Harmony primarily for women navigating hormonal changes — sleep disruption, brain fog, mood shifts — but the D3+K2+Magnesium combination is something I think most Irish adults could benefit from, particularly from September onwards when sunlight becomes scarce.
What are the best ways to support your vitamin D levels for better sleep?
Given our climate, this takes a bit of strategy. Here's what I actually do and recommend:
- Supplement from September to March. The HSE recommends 10–15 micrograms (400–600 IU) daily, but many practitioners suggest 1,000–2,000 IU D3 for adults — especially if you have confirmed deficiency. Ask your GP to test your levels if you're unsure.
- Take it in the morning, not at night. There's evidence that vitamin D taken in the evening can interfere with melatonin production, given its role in circadian regulation. Morning with a meal is ideal, since D3 is fat-soluble.
- Get outside midday in summer. Between May and September, 15–20 minutes of midday sun on your arms and face without sunscreen triggers meaningful D3 synthesis — even on partially cloudy days.
- Eat vitamin D-rich foods. Oily fish (salmon, mackerel, sardines), egg yolks, and fortified dairy all contribute. They won't replace supplementation in Ireland, but they help maintain levels.
- Pair with magnesium. Magnesium is essential for vitamin D activation. Since many Irish people are low in both, addressing them together makes practical sense.
If you're also dealing with racing thoughts at night or finding it hard to switch off, it's worth knowing that low vitamin D can contribute to elevated cortisol — and cortisol is one of the biggest disruptors of sleep onset. I wrote about that connection in more detail here: What's the Connection Between Cortisol and Sleep?
Frequently asked questions about vitamin D and sleep
Does low vitamin D cause insomnia?
Low vitamin D doesn't directly cause insomnia, but it significantly disrupts the systems that regulate sleep — including melatonin production and circadian rhythm gene expression. Studies show that people with deficiency have a roughly 50% higher risk of sleep disorders, including insomnia. For many people, it's a missing piece they hadn't considered.
How long does it take for vitamin D to improve sleep?
Most people notice changes in sleep quality within 4–8 weeks of consistent supplementation, though this varies depending on how deficient you were to begin with. Blood levels typically stabilise after 2–3 months. Combining D3 with K2 and magnesium tends to produce more comprehensive results than D3 alone.
Is it safe to take vitamin D every day?
Yes, daily supplementation at 1,000–2,000 IU D3 is considered safe for most healthy adults. Toxicity is rare and only occurs at very high doses over a prolonged period. If you have any health conditions or take medications, check with your GP before starting.
Why do Irish people have such low vitamin D levels?
Ireland's northerly latitude (51–55°N) means the sun's angle is too low for effective UVB radiation for roughly half the year — from October through April. During these months, no amount of time outdoors will trigger meaningful vitamin D synthesis in the skin. Combined with indoor lifestyles and dietary patterns, deficiency is very common here.
Can vitamin D help with sleep during perimenopause?
This is an area I find particularly important. Oestrogen influences vitamin D receptor sensitivity, so as oestrogen declines during perimenopause, vitamin D metabolism can become less efficient — affecting melatonin production and sleep architecture. Supporting vitamin D levels alongside magnesium and other key nutrients can make a meaningful difference for women in this stage of life.
If you've been struggling with sleep and haven't checked your vitamin D levels recently, I'd genuinely encourage you to ask your GP for a simple blood test. For many people I speak with, it turns out to be a key piece they hadn't considered.
In the meantime, if you'd like to support your levels with a formula designed for Irish women — combining D3+K2, magnesium bisglycinate, lion's mane, zinc, and B vitamins — take a look at Harmony. It's what I reach for myself, especially through the autumn and winter months.
This article is for informational purposes only and does not constitute medical advice. Please consult your GP if you are experiencing persistent sleep difficulties.
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Does Vitamin D Affect Sleep? What Every Irish Adult Needs to Know