Vitamin D and Sleep: Why I Was Low Living in Cyprus

I live in Cyprus. There is sun here in a quantity that northern Europeans find difficult to believe, and for a long time I assumed that settled the vitamin D question entirely. I take a supplement anyway, mostly out of habit from Germany, and I haven’t expected a blood test to tell me I had been wasting my money.

It came back at 33 ng/mL. Technically fine, sitting just above the threshold most guidelines use for sufficiency, and nowhere near the comfortable number I had expected from someone living on a Mediterranean island.

My first assumption was that it was the training and the sweating. That turned out to be the wrong answer. The right one is more interesting, and it led me straight into the research on vitamin D and sleep.

The Mediterranean Paradox

This is where it stopped being about me.

A population study published in Scientific Reports in 2022 looked at vitamin D measurements from over 519,000 people in a Mediterranean region. Around 75 percent had suboptimal levels, meaning at or below 30 ng/mL, and among young people that figure rose to roughly 80 percent.

A separate study in Athens, one of the sunniest cities in Europe, found 92 percent of the sample below the 30 ng/mL mark, with nearly 40 percent severely deficient.

And here is the part that genuinely surprised me. Vitamin D deficiency is more common in southern Europe than in northern Europe, despite the ultraviolet radiation being far more effective in the south. The sunshine is not the limiting factor.

So my level is not an anomaly. It is roughly what living in the Mediterranean actually produces.

Why Sun Does Not Automatically Mean Vitamin D

The researchers point to a cluster of reasons, and none of them is sweating.

People in hot countries avoid the midday sun. This is the one I recognised immediately. When it is 38 degrees outside, nobody is standing in direct sunlight between eleven and three.

You walk on the shaded side of the street. You stay inside. The hours with the strongest UVB, which is the part that actually produces vitamin D in skin, are exactly the hours people here plan their day around avoiding.

Sun protection. The Scientific Reports authors list the use of sun protection among the probable causes at a population level. I want to be careful here, because the evidence is genuinely mixed. One study in Lebanon found sunscreen users had slightly lower deficiency rates, which the authors attributed to other habits of people who use sunscreen rather than to the sunscreen itself. So the picture is not as clean as “sunscreen causes deficiency.”

That said, I wear SPF on my face and décolleté every single morning, which I wrote about in the sleep and skin post, and I am not going to stop. Sun protection has the strongest evidence of anything in my routine for how my skin ages. A supplement is a far cheaper way to solve the vitamin D side than giving up sunscreen.

Diet and no fortification. Northern European countries fortify foods with vitamin D. Mediterranean countries largely do not, and the diet is not naturally high in it. Oily fish, egg yolks and not much else.

Training indoors. If your workouts happen in a gym rather than outside, which mine do, that removes the most likely window of sustained daylight exposure from your day.

Vitamin D and Sleep: What the Research Shows

This is where vitamin D and sleep come together, and it is what made this relevant for Driftly rather than just personally interesting.

A meta-analysis published in Nutrients pooled nine studies covering more than 9,000 adults and found vitamin D deficiency associated with a higher risk of sleep disorders. Specifically, the odds of short sleep duration were around 1.74 times higher, and the odds of poor sleep quality around 1.59 times higher, in people who were deficient.

That is an association between vitamin D and sleep rather than proof of cause, and the authors said so plainly, calling for more cohort studies and randomised trials.

The supplementation side is where I want to be careful, because this is usually where supplement marketing runs ahead of the data. A 2022 systematic review and meta-analysis of intervention studies, also in Nutrients, found that supplementation improved sleep quality scores measured on a standard sleep index. The authors concluded that supplementation is promising for sleep quality, and that its effect on sleep quantity and on specific sleep disorders needs further investigation.

Promising is the operative word. Not proven.

What I Took From This

I am keeping the supplement, and after reading up on vitamin D and sleep I am clearer about why.

The reasoning is not that vitamin D will transform my sleep. The evidence does not support that claim, and anyone making it is going further than the research allows. What actually moves my sleep is still the boring stuff, like my evening routine. The reasoning is that low vitamin D is consistently associated with worse sleep, that three quarters of the population around me is below the sufficiency line, and that correcting a likely deficiency is cheap and low risk when done at a sensible dose.

I use a vitamin D3 supplement daily through the year rather than seasonally, because the Mediterranean data suggests the summer sun is not doing the job people assume it is.

The one thing I would genuinely recommend to anyone reading this: get the blood test before deciding anything. Vitamin D is fat soluble, which means it accumulates rather than being flushed out, and guessing at a dose is the wrong approach. An at home vitamin D test is enough to tell you whether you are in the conversation at all, and a doctor can interpret it properly from there.

Frequently Asked Questions

Does vitamin D actually improve sleep?

The association between vitamin D and sleep is reasonably well established. Deficiency is linked to shorter sleep and poorer sleep quality across multiple observational studies. Whether supplementing improves sleep in someone who is already sufficient is much less clear, and the reviews describe the evidence as promising rather than settled. If you feel tired all the time, low vitamin D is only one of several reasons worth ruling out.

Can you be deficient in a sunny country?

Yes, and it is common. Studies across Mediterranean and Middle Eastern populations consistently find high rates of insufficiency despite abundant sunshine, driven mostly by sun avoidance during peak hours, time spent indoors, and diets low in vitamin D without food fortification.

Does sweating lower your vitamin D?

This comes up often and I could not find good evidence for it. The better documented reasons athletes are often low are training indoors, limited daylight exposure during the working day, and higher demand rather than loss through sweat.

What level should I be aiming for?

Guidelines disagree, which is part of the confusion. Some bodies use 20 ng/mL as the sufficiency cutoff and others use 30 ng/mL. That is a question for your doctor with your actual results in front of them rather than something to settle from a blog.

Should I take vitamin D at night or in the morning?

There is no strong evidence either way for the vitamin itself. Some people report that taking it late disrupts their sleep, though this is anecdotal rather than well studied. Taking it with a meal containing fat matters more for absorption than the time of day does.

The Bottom Line

I expected living in Cyprus to make the vitamin D question irrelevant. The data says the opposite. Deficiency is more common in sunny southern Europe than in the cloudy north, because the limiting factor is behaviour rather than available sunlight.

The link between vitamin D and sleep is real but modest. Low vitamin D travels with shorter and poorer sleep across a lot of studies. Supplementing improves sleep scores in intervention research, though the authors themselves call it promising rather than proven.

My level was 33. Technically fine, lower than I assumed, and higher than three quarters of the people around me manage. I am keeping the supplement and the sunscreen both.

Have you ever had your vitamin D tested, and did the number surprise you? I would genuinely like to know!