Around 90% of Irish adults aren't getting enough vitamin D. And here's something that doesn't get talked about nearly enough — that deficiency isn't just making you tired in the afternoon. It might be quietly disrupting your sleep from the inside out.

I started looking into this after a particularly rough winter a few years back. I was doing everything "right" — decent bedtime routine, no caffeine after 2pm, the lot. And still, I was waking up groggy, struggling to fall asleep, and feeling like I hadn't recovered by morning. When my GP suggested checking my vitamin D levels, I honestly thought it was a long shot. It wasn't. Mine were through the floor.

Does vitamin D actually affect your sleep?

Yes — and the research on this has grown significantly in recent years. Vitamin D isn't just about bones and immunity. Scientists have found vitamin D receptors (VDR) in multiple areas of the brain directly involved in sleep regulation, including the hypothalamus, which is essentially your body's sleep-wake command centre.

A meta-analysis published in the journal Nutrients found that vitamin D supplementation significantly improved sleep quality scores across multiple trials. A separate large study found that people with vitamin D deficiency were significantly more likely to report poor sleep quality than those with sufficient levels — 21.7% versus 19.8%, a meaningful gap when you're talking about population-wide numbers.

One key mechanism is the role vitamin D plays in melatonin production. It activates an enzyme called TPH1, which helps convert serotonin into melatonin in the evening hours. If your vitamin D is low, this conversion can be disrupted — meaning your body may produce less melatonin at the times it needs it most. Vitamin D also regulates core circadian rhythm genes including Clock, Bmal1, and Per2, which are the genes that keep your internal body clock running on time.

Why are so many people in Ireland vitamin D deficient?

This is something I see a lot — and if you're living in Ireland, it genuinely isn't your fault. Ireland sits between 52 and 55 degrees north latitude, which means we only get sufficient UVB sunlight to synthesise vitamin D from around April through to October. For the remaining six months — what researchers call a "vitamin D winter" — we're entirely dependent on diet and supplementation.

And Irish diets aren't set up well for this. The main food sources are oily fish, egg yolks, and fortified foods, which most people aren't eating in sufficient quantities. Research from University College Dublin found that approximately half of the Irish population has vitamin D levels below 50 nmol/l, and around 90% of adults have inadequate intakes. These aren't fringe numbers — this is the majority of the population walking around deficient for a significant part of the year.

So if you're waking up feeling unrested, dragging through the winter months, and wondering why your sleep never quite feels restorative — low vitamin D is worth taking seriously.

What does vitamin D actually do in the brain during sleep?

Vitamin D receptors are found in the hypothalamus, substantia nigra, hippocampus, and prefrontal cortex — brain regions that directly or indirectly regulate sleep. This isn't coincidence. Researchers now believe vitamin D influences sleep through several overlapping pathways.

First, as mentioned, it supports melatonin synthesis. Second, it regulates brain-derived neurotrophic factor (BDNF), a protein involved in sleep homeostasis — essentially how your brain builds up and releases sleep pressure throughout the day. Third, vitamin D has anti-inflammatory effects, and chronic low-grade inflammation is increasingly linked to disrupted sleep architecture.

Here's what I put in Harmony and why: I included Vitamin D3 paired with K2 (as MK-7). These two nutrients work in tandem — K2 ensures that calcium gets directed where it's needed (bones and teeth) and stays away from soft tissue, which matters when you're supplementing D3 consistently. D3 alone raises your levels; D3 with K2 does so more safely and effectively. This pairing is used in the strongest research, and it's the version I'd always recommend over D3 in isolation.

What can you do right now to support your vitamin D levels and sleep?

Here's what the evidence supports — and what I do myself, particularly as we head into autumn:

Start supplementing in September, not December. Most people wait until they're clearly deficient before acting. But vitamin D levels start declining in early autumn even after a decent summer. Get ahead of it.
Always pair D3 with K2. This is the combination backed by research and the one I'd recommend for anyone supplementing long-term.
Get your levels tested. A simple blood test (serum 25(OH)D) from your GP will tell you exactly where you stand. Optimal levels for sleep support are generally considered to be 75–125 nmol/l.
Add magnesium to the mix. Magnesium is required for vitamin D to be converted into its active form in the body. If your magnesium is low, you may not be getting the full benefit of your D3. Our Magnesium Bisglycinate is one of the most bioavailable forms available.
Give it time. Vitamin D levels don't recover overnight. Most people need 8–12 weeks of consistent supplementation before seeing meaningful changes in their serum levels — and their sleep.

If you've been struggling with a racing mind keeping you awake as well as poor sleep quality, I wrote about the neuroscience of that here: How to Stop Racing Thoughts at Night: What the Science Says. And for the broader picture of what's driving poor sleep in Ireland specifically, this is worth a read: Why Can't Irish People Sleep? A Science-Backed Guide to Better Rest.

Frequently asked questions

Does vitamin D help with insomnia?

Research suggests that vitamin D deficiency is linked to a higher risk of insomnia and disrupted sleep. A meta-analysis of intervention studies found that vitamin D supplementation significantly improved sleep quality scores. It's not a sleeping tablet, but addressing deficiency — especially common in Ireland — may meaningfully support better sleep, particularly when combined with other sleep-supportive nutrients like magnesium and L-Theanine.

What time should I take vitamin D for better sleep?

Most researchers suggest taking vitamin D in the morning rather than at night. Because vitamin D influences circadian rhythm signalling, taking it later in the day may interfere with melatonin production. Consistency matters more than timing, but morning is generally the better window.

Can low vitamin D cause tiredness even after a full night's sleep?

Yes — low vitamin D is associated with non-restorative sleep, meaning you can sleep eight hours and still wake up exhausted. It's also linked to daytime fatigue, low mood, and brain fog. In Ireland, where deficiency is widespread, it's one of the first things worth ruling out if you're chronically unrested despite decent sleep habits.

How long does it take for vitamin D to improve sleep?

It typically takes 8–12 weeks of consistent supplementation to raise serum vitamin D levels meaningfully. Some people notice improvements in energy and mood before then. For sleep quality specifically, give it at least two to three months before assessing the impact — and ensure you're combining D3 with K2 for best results.

Is vitamin D3 or D2 better for sleep?

D3 (cholecalciferol) is significantly more effective at raising and maintaining serum vitamin D levels than D2 (ergocalciferol), and is the form used in the majority of sleep-related research. Paired with K2 (as MK-7), D3 is both more effective and safer for long-term supplementation.

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If you've tried the usual sleep advice — the bedtime routines, the no-screens rule, the chamomile tea — and you're still not sleeping well, this could genuinely be the missing piece. Particularly here in Ireland, where most of us are deficient for a significant chunk of the year. I'd encourage you to get your levels tested if you haven't already.

And if you're looking for a supplement that includes Vitamin D3+K2 alongside other nutrients women in particular tend to need — Lion's Mane for brain fog, Magnesium Bisglycinate for sleep and nervous system support, and B vitamins for energy — that's exactly what I built Harmony for. It's designed for the whole picture, not just one piece of the puzzle.

This article is for informational purposes only and does not constitute medical advice. Please consult your GP if you are experiencing persistent sleep difficulties or if you are considering vitamin D supplementation at high doses.