It's in the Rhythm of the Light
What Aotearoa’s Light Environment Teaches Us About Human Biology: From Vitamin D to Skin Health, Immunity and Mood
Light. Something pleasant. Something to block. Something to switch on or off.
There’s a tendency now to treat artificial light as equivalent to natural light. We blur the difference between the two, confuse the mechanisms, and then wonder why our sense of feeling healthy feels, well, somewhat off.
Human evolution depends on sunlight as information. Light told the body when
to be alert, when to rest, when to repair, and how to move through the seasons.
That relationship still exists, even though we pay far less attention to it.
Vitamin D sits inside this light-biology network. It’s often described as a simple nutrient, but biologically it functions more like a hormone precursor. Rather than acting directly, it helps cells decide what to prioritise: repair, immune defence, inflammation control and energy production. It does this by influencing which instructions inside the cell are read and acted on at any given time.
Vitamin D wasn’t isolated until the early 1900s. Its hormonal role wasn’t understood until the 1970s. Much of the research linking Vitamin D to circadian timing, mood and immune function has emerged only in the last few decades.
Vitamin D is a critical support molecule for human biology, but it works within a larger light-regulated system rather than acting as a solution on its own.
The body makes Vitamin D primarily when UV-B light hits the skin. Food contributes small amounts. Supplements can help in winter, but they don’t replace the broader biological signals sunlight delivers through both the skin and the eyes.
This becomes particularly relevant in Aotearoa, where winter UV-B often falls below the level needed for meaningful Vitamin D synthesis. Across much of the country, especially from the lower North Island southward, there are extended periods in winter when the skin simply cannot make Vitamin D, even on bright, clear days.
This seasonal Vitamin D gap overlaps with reduced daylight reaching the eyes, compounding effects on mood, sleep timing and immune regulation. By late winter, many people feel the downstream effects: heavier mornings, flatter mood, slower physical recovery, reduced stress tolerance, and a general sense that everything takes more effort.
Aotearoa acts almost like a stress test for human light-regulated biology. The extremes here exaggerate patterns that are present in many parts of the world.
Unlike places where winter UV loss is balanced by summers that are more gentle, New Zealand concentrates both ends of the spectrum into a single environment.
For some, this seasonal shift becomes pronounced enough to be diagnosed as Seasonal Affective Disorder. But the underlying mechanisms are the same ones that affect everyone to varying degrees. Reduced daylight to the eyes alters circadian timing, serotonin and dopamine tone shifts, melatonin drifts later, cortisol loses its rhythm, appetite changes, and the whole system slows. Declining Vitamin D adds another layer of load.
But this post isn’t just about Vitamin D or SAD. To understand why light affects people so strongly, especially here, we need to look at the whole system. How the retina sets hormonal timing. How the skin prepares itself for UV. Why sunglasses change circadian signalling. How different skin tones interact with sunlight in Aotearoa. And how modern habits interrupt signals the body still relies on.
New Zealand’s extreme seasonal contrast, with high summer UV followed by a sharp winter decline, exposes the full range of light conditions the body must adapt to. When indoor work, sunscreen habits and frequent use of sunglasses are added, those signals become fragmented, making it harder to maintain a stable biological rhythm.
The sections ahead clearly map out this system, including the parts that are rarely explained properly, while clarifying the signals your body already responds to instinctively.
How the Body Reads Light: Eyes, Skin, Timing and Sunglasses
Human physiology depends on light more than most people realise. Morning daylight reaching the eyes sets the timing for mood, energy, sleep and metabolic rhythm. UV-B reaching the skin activates photoprotective chemistry and initiates Vitamin D synthesis. These are not optional processes. They are baseline operating instructions the body still relies on, even as daily life has shifted indoors.
The body doesn’t experience light as brightness alone. It reads light through two primary pathways: the eyes and the skin. Each delivers different information, and each influences different systems.
Morning daylight entering the eyes helps set internal timing. It anchors the daily rhythm that governs alertness, energy, mood, sleep timing and metabolic pace. Separately, UV-B reaching the skin triggers a set of chemical responses that support photoprotection and initiate Vitamin D synthesis.
These pathways work together, but they are not interchangeable. Light to the eyes cannot replace light to the skin, and Vitamin D alone cannot substitute for the timing signals delivered through vision. When either pathway is disrupted, the system loses coherence.
How Morning Light Sets the Internal Clock
The eyes do more than see. Inside the retina are specialised photoreceptors that respond strongly to natural daylight, particularly the blue-rich light present in the morning. Their signal travels directly to the brain’s central timing centre, the suprachiasmatic nucleus, which coordinates the daily rhythm of sleep and wakefulness, hormone release, mood, appetite, immune activity and energy levels.
This system depends on light intensity. Indoor lighting rarely reaches the levels needed for full activation. Even a cloudy morning in Aotearoa can deliver tens of thousands of lux, far more than most indoor environments. Just five to fifteen minutes of unfiltered morning daylight is often enough to stabilise circadian timing for the day ahead.
When this signal arrives at the right time, the body responds in a predictable sequence. Serotonin begins to rise, dopamine tone increases, melatonin is suppressed early rather than lingering into the morning, and cortisol reaches a clear peak that helps organise energy and focus. When the signal is weak, delayed or filtered, that timing becomes less precise, and the effects ripple across the day.
What the Skin Is Doing at the Same Time
While the eyes are setting internal timing, the skin is running its own set of light-responsive processes.
When UV-B reaches the outer layers of skin, a sequence of protective responses begins:
Compounds in the epidermis shift into forms that help moderate immune activity and reduce UV damage.
Keratinocytes release signals that activate melanin pathways.
The epidermis gradually thickens.
Melanocytes distribute pigment across cell nuclei, forming a physical shield that protects DNA from excess radiation.
This is the body’s built-in photoprotection system. It only activates in response to real sunlight, and it works best with gradual, repeated exposure across days and weeks. Sudden, high-intensity bursts overwhelm the system before it has time to adapt.
Vitamin D synthesis begins in this same layer of skin. UV-B converts
7-dehydrocholesterol into Vitamin D precursors, linking sunlight directly to immune regulation, inflammation control and cellular energy. Supplements can support Vitamin D levels when UV-B is absent, but they don’t reproduce the broader photochemical responses that occur when skin and sunlight interact directly.
Where Sunglasses Disrupt This System
The bad news is that sunglasses can interrupt the light signal the brain relies on to set circadian timing. The good news is that you don’t need to stop wearing them. You just need to be more deliberate about when you wear them.
Upgrade to a Paid Subscription for the full post covering more topics including:
How the Skin Prepares for Sunlight.
The Paradox of More Time Spent Indoors.
People, Place and the Light: Why Skin Behaves Differently Here.
Winter Vitamin D: Limits, Supports and Reality + more.