By Michelle Boren, Founder of SacredLee
I have vivid memories of getting my first blood panel, long before I understood that vitamin D works less like a vitamin and more like a sunlight hormone made in the skin. My father took me, which is probably why it became such a lasting core memory. My dad did not believe in sick days. Rain, snow, flu, blizzard, none of it mattered. He went in.
I'm not entirely sure what landed me in that chair. My best guess is that it had something to do with the fact that I was a sickly child, always the first to catch a cold, the one up all night coughing, the kid who would suddenly break out in mystery hives. Whatever the reason, I was six or seven years old, and somehow I hit the jackpot: a day off from school, a prize from the treasure box, and my dad all to myself. I remember starting to cry when the nurse brought out the needle, and my father doing a little jig behind her back to make me laugh.
I don't remember most of what came back on that blood panel. But I remember one thing very clearly: I had high cholesterol. In our house, that was a big deal. I was a child of the '80s, when brightly colored boxes promising FAT FREE! and NO CHOLESTEROL! seemed to land on grocery store shelves every month, and money was tight enough that 99 percent of what I enthusiastically tossed into the cart got quietly put back. High cholesterol changed that math. Suddenly I had medical justification, and the SnackWells could stay. (For the record, I wasn't a chubby kid. That didn't arrive until somewhere in the middle of high school.)
The cholesterol clearly ran in the family, and it's something I've paid attention to ever since. Years later, I learned that low vitamin D was another pattern running through my mother's side, woman after woman, and now my son as well. Looking back, other pieces of that family story make more sense too. I don't think my mother has slept through an entire night since the 1970s.
For most of my life, I thought about vitamin D the way most of us were taught to: bones, maybe calcium, maybe sunshine, and not much else. I had no idea it was connected to sleep. I didn't know researchers had identified vitamin D receptors in the parts of the brain that govern sleep-wake regulation. And I certainly didn't understand that what we casually call a vitamin is biologically much closer to a hormone. That is what sent me down a very different rabbit hole, because once you understand what vitamin D really is, the conversation becomes much bigger than bones.
Why Is Vitamin D Really a Sunlight Hormone?
Vitamin D is called a vitamin mostly out of habit. Real vitamins are nutrients we have to get from food because the body can't make enough on its own. Vitamin D breaks that rule. Your skin makes it, starting the moment UVB light hits a cholesterol-derived molecule called 7-dehydrocholesterol. From there it travels to the liver, where it becomes 25-hydroxyvitamin D (the form your blood test measures), and then to the kidneys, where it becomes the active form, calcitriol, a genuine steroid hormone that moves through your bloodstream and talks to receptors in tissue all over your body.
That is a very different story than "the sunshine vitamin." It's less a nutrient you're topping off and more a communication system between light, skin, and your genes. The NIH still classifies it as a vitamin for nutritional purposes, and that's fine, both things can be true. But once you see the hormone side, you can't unsee it.
Who Discovered Vitamin D Receptors in the Brain?
For decades, vitamin D research centered almost entirely on bones, calcium, and rickets. Then researchers started finding vitamin D receptors in places that made no sense under that story, and one of the scientists who helped change the conversation was Walter E. Stumpf. In 1982, Stumpf and colleagues published work in Science showing specific target sites for active vitamin D in neurons of the brain and spinal cord in animal models, well beyond calcium and bone.
Stumpf kept mapping receptor sites for years afterward, including a 1987 autoradiographic study in Histochemistry pinpointing specific brain targets, and helped advance the idea of vitamin D as a true hormone with effects throughout the body. He also got interested in seasonality: his animal research found differences in vitamin D receptor activity across species tied to seasonal changes, feeding his hypothesis that vitamin D might be part of how the body responds to sunlight, season, reproduction, metabolism, and behavior. This is where the popular idea of vitamin D as a "hibernation hormone" comes from, though I would not take that phrase literally. Humans do not hibernate. But the larger question Stumpf raised still holds up: could vitamin D be one of the ways the body reads the changing availability of sunlight?
Does Vitamin D Affect Sleep?
This is where Dr. Stasha Gominak enters the story. A neurologist, she began noticing low vitamin D levels in many patients with abnormal sleep and neurological complaints, and connected those observations with Stumpf's earlier work identifying vitamin D target neurons in brain areas involved in sleep regulation. In 2012, Gominak and Stumpf published a hypothesis proposing that widespread vitamin D deficiency could be contributing to sleep disorders, drawing on an uncontrolled clinical experience involving roughly 1,500 patients, many of whom said their sleep improved as their vitamin D status changed. Because it wasn't a randomized controlled trial, it can't establish cause and effect, and their proposed "ideal" blood range shouldn't be read as universal medical consensus.
The story didn't stop there. A 2022 systematic review examining 19 vitamin D intervention studies, including 13 randomized controlled trials, found that supplementation appeared promising for sleep quality, with moderate certainty of evidence, though effects on sleep duration and specific disorders were less consistent. A second 2022 meta-analysis found the same pattern.
So I think we can say this much clearly: vitamin D and sleep are connected. What researchers are still working out is exactly how, which sleep problems respond most, and whether deficiency is a cause, a consequence, or part of a larger pattern. For me, it makes my mother's lifelong sleep struggles and our family's low vitamin D pattern a lot more interesting. Not proof. A reason to pay attention. If sleep is a persistent struggle in your own life, Non-Toxic Bedroom Essentials covers the environmental side of that conversation.
Is Vitamin D Only About Bone Health?
None of this diminishes vitamin D's original claim to fame. It's essential for calcium absorption and bone mineralization, and severe deficiency can cause rickets in children and osteomalacia in adults. That part of the story is settled biology.
But vitamin D receptors show up in tissues throughout the body, and research has expanded into muscle function, immune signaling, inflammation, reproductive health, neurological function, and more. That doesn't mean vitamin D is a cure-all, and this distinction matters: finding a receptor in a tissue tells us the hormone can communicate there. It doesn't mean taking more vitamin D will prevent or cure every condition tied to that tissue. The wellness world sometimes skips that middle step. SacredLee should not. What the receptor map does tell us is that the old view of vitamin D as simply "the thing you need for bones" was incomplete.
Does Vitamin D Affect Mood and Depression?
Mood is another place where the relationship is hard to dismiss, but it still requires nuance. Low vitamin D status has repeatedly been linked to depressive symptoms in observational research, though that alone can't tell us which direction the relationship runs. Someone experiencing depression may spend less time outdoors, move less, eat differently, or sleep differently, and seasonality can influence both mood and vitamin D production at the same time.
But randomized trials of vitamin D supplementation add another layer. A 2024 systematic review and dose-response meta-analysis of 31 randomized trials involving more than 24,000 adults found that supplementation was associated with reductions in depressive symptoms, particularly over shorter treatment periods and among people already experiencing symptoms, though the authors still called for more high-quality research. Other meta-analyses have found similar benefits, though effect size and evidence quality vary.
So again, the right conclusion isn't "vitamin D cures depression." It doesn't. The better question is why sunlight, season, vitamin D status, sleep, and mood so often show up in the same biological conversation. I find that question far more interesting.
The Modern Indoor Experiment
For most of human history, daylight was not something we scheduled. It was the environment. Then, in an astonishingly short period of evolutionary time, we moved life indoors. We work indoors, children learn indoors, we exercise and shop indoors, and drive from one indoor environment to another, while screens let activity, entertainment, and work continue long after daylight disappears.
None of this means computers "caused" vitamin D deficiency, and I don't think we need to manufacture a neat historical date when everything suddenly went wrong. But our relationship with daylight has unquestionably changed.
Glass also matters. You can sit beside a beautiful sunny window all day and receive visible light, but ordinary window glass blocks most UVB radiation, the wavelength needed for meaningful vitamin D production in the skin. So "being in the sun" and having UVB reach your skin are not always the same thing. We built environments that let us live almost entirely detached from outdoor light. Our biology is considerably older than those environments.
Does Sunscreen Cause Vitamin D Deficiency?
This is where the conversation can get unnecessarily polarized. UVB radiation is required for vitamin D production in skin, and sunscreen is designed to reduce UV exposure, so in theory, sufficiently applied sunscreen can reduce vitamin D synthesis. But real life isn't a laboratory. A systematic review examining sunscreen use and vitamin D found that experimental studies show an effect under artificial conditions, while field trials and observational research generally have not shown ordinary sunscreen use to cause vitamin D deficiency. People don't apply sunscreen perfectly. We miss spots, skip reapplication, and pick up incidental sun exposure throughout an ordinary day regardless.
So I don't think the responsible message is stop wearing sunscreen so you can make vitamin D, and I don't think the answer is to treat sunlight as inherently dangerous either. The body needs light. Skin also needs protection from burning and cumulative UV damage. Both are true at once.
How Much Sun Do You Need?
This is one place where wellness loves a simple number. "Fifteen minutes a day." "Twenty minutes at noon." I wish it were that simple. Vitamin D production through the skin varies dramatically depending on latitude, season, time of day, skin pigmentation, age, cloud cover, air pollution, how much skin is exposed, sunscreen use, and individual biology. A fair-skinned person outdoors in Florida in July is having a very different UVB experience from a deeply pigmented person in Boston in January. That is why generic sunlight prescriptions are not especially useful. Regular outdoor time matters for many reasons, but if you want to know your vitamin D status, test it rather than guessing from how much sun you think you get.
Does Skin Pigmentation Affect Vitamin D Production?
Melanin is protective. It absorbs ultraviolet radiation and helps protect skin from damage, but it also means darker skin generally needs more UVB exposure to produce the same amount of vitamin D as lighter skin under equivalent conditions. This is one reason people with darker skin living in northern climates can be at greater risk of vitamin D inadequacy. It is not a flaw in darker skin. It is biology interacting with geography, a trait beautifully suited to one light environment behaving differently when humans move across latitudes faster than evolution does.
Can You Get Enough Vitamin D From Food?
Vitamin D is unusual here too. Relatively few foods naturally contain meaningful amounts. Fatty fish such as salmon, trout, mackerel, and sardines are among the better natural sources, along with egg yolks, mushrooms exposed to ultraviolet light (which provide vitamin D2), and fortified milks, plant milks, and cereals.
It's worth pausing on where that fish and egg-yolk vitamin D comes from in the first place. Fatty fish accumulate theirs through the food chain: phytoplankton make it from sunlight at the ocean's surface, it moves up through zooplankton and smaller fish, and it concentrates in fatty tissue and liver by the time it reaches your plate. Hens pass vitamin D into the yolk based on their own diet and sun exposure. In both cases, you are eating vitamin D that another organism already made or accumulated, not vitamin D your own skin produced. Mushrooms are the interesting exception. Exposed to ultraviolet light, they synthesize vitamin D2 themselves, the same basic process your skin uses to make D3, just triggered in a different organism.
And then there's cod liver oil, used for generations by families in northern climates during the darker months. They may not have understood receptor biology, but they knew something about winter. It provides vitamin D, omega-3 fatty acids, and vitamin A, but traditional doesn't mean unlimited: it can also carry substantial preformed vitamin A, so quality and dose matter, particularly during pregnancy. This is one of those places where the old remedy may still have value, but discernment matters more than nostalgia.
What Does a Vitamin D Blood Test Measure?
When your doctor checks your vitamin D, they're testing 25-hydroxyvitamin D, or 25(OH)D for short. That's the storage form circulating in your blood, and it's the best routine marker of where you stand. It is not the active hormone. The active form, calcitriol, is tightly regulated by your body and isn't the one your doctor typically tests. That's why people get confused: you can be running low in the storage form while your body still holds active hormone levels steady, at least for a while. Your body is remarkably good at prioritizing what it needs most. Until it isn't.
What Number Should You Aim For?
This is where I want SacredLee to resist another wellness trap: there's no universally agreed "perfect" vitamin D number. Different medical organizations define deficiency, insufficiency, and adequacy somewhat differently. The NIH considers serum 25(OH)D levels of 20 ng/mL or higher sufficient for most people for bone and overall health, with anything below 12 ng/mL generally too low, while other clinicians and researchers advocate different targets depending on the individual and the condition in question. Gominak's own work has argued for a substantially higher, narrower range in the context of sleep specifically, and that should be read as her clinical hypothesis, not universal consensus. This is why I trust testing, context, and thoughtful medical guidance over chasing an internet number. More is not automatically better.
More Vitamin D Is Not Better Vitamin D
Vitamin D is fat soluble, which means excessive supplementation can accumulate and become toxic. Too much can produce dangerously high calcium levels and lead to nausea, weakness, kidney problems, abnormal heart rhythms, and other complications. The NIH specifically warns that toxicity generally comes from excessive supplementation, not sun exposure, because the body has built-in mechanisms that limit how much it makes from sunlight. A supplement bottle does not. That is another reason to know your level before taking large doses for long periods of time. The goal is adequacy. Not winning vitamin D.
Vitamin D Is Not the Same Thing as Sunlight
This may be the most important distinction in the entire article. A vitamin D supplement can raise your vitamin D level. It cannot replace the sun. Daylight reaches the eyes and helps set our circadian clock, outdoor light influences sleep-wake timing, being outside generally means moving more, and nature affects stress and attention in ways a capsule cannot. Vitamin D is one product of our relationship with the sun. It is not the entire relationship. I think wellness sometimes makes the mistake of discovering one molecule involved in a natural process and then assuming we can bottle the molecule and discard the process. Biology is rarely that simple.
Where You Have Agency
Know your level. If vitamin D deficiency runs in your family, you have symptoms or risk factors, or you spend very little time outdoors, ask whether testing makes sense for you.
Go outside regularly, not just for vitamin D but for light, movement, circadian rhythm, and everything else outdoor life provides.
Use food where it makes sense. Fatty fish, eggs, fortified foods, UV-exposed mushrooms, and thoughtfully chosen cod liver oil can all contribute.
Supplement when you need to, with a dose that reflects your starting level, individual needs, diet, sun exposure, and medical context.
Protect your skin intelligently. Avoid burning, and use sunscreen and protective clothing when appropriate.
Pay attention to sleep. If sleep is poor and vitamin D is low, that is worth raising with your practitioner, especially given the growing research connecting the two. How to Support Your Body's Detox Pathways covers the sleep-and-liver side of that same conversation.
You don't need to chase the sun or megadose supplements. Start with information. The goal is not perfection. It is understanding enough about your own biology to make better choices.
Closing Reflection
I keep coming back to what an extraordinary system this is. Light hits the skin. The body turns that signal into chemistry. The liver changes it, the kidneys activate it, and a hormone travels through the bloodstream to speak with receptors throughout the body, including the brain. For most of my life, I thought vitamin D was just something that helped bones.
Maybe that's what I find so humbling about learning how the body works. We keep discovering that what looked simple was not simple at all. Vitamin D may ultimately teach us something bigger than whether to take another supplement: that the human body developed in relationship with the natural world, with seasons, darkness, morning light, and the sun on our skin.
We've become extraordinarily good at building environments that shield us from nature: temperature-controlled rooms, artificial light, screens that let day continue indefinitely, cars that carry us from garage to garage without really stepping outside. There are wonderful things about modern life. But our biology still remembers something older, and perhaps the lesson of vitamin D isn't simply that we need more of it. Perhaps it is that the body still expects a relationship with the sun. That may be worth remembering.