From a strictly biological perspective, humans evolved in a world ruled by the Sun. Long before artificial light, before enclosed rooms and climate control, our metabolism was calibrated to a 24-hour light–dark cycle driven by solar radiation.
Plants respond to photoperiodism: flowering, dormancy, growth.
Bees read ultraviolet patterns on petals.
Birds migrate according to daylight length.
Mammals regulate melatonin and cortisol based on sunrise and sunset.
We are mammals. We are not exempt.
Circadian rhythms determine when we sleep, when hormones rise and fall, how we respond to winter and spring. Even reproductive hormones shift with seasonal light exposure. For most of evolutionary history, solar radiation was not optional input. It was regulatory architecture.
Now we live indoors. We sleep behind light-blocking curtains. We work under artificial illumination. We avoid midday sun. We apply sunscreen reflexively. We remain awake long after sunset. In short, we have detached ourselves from the environmental signal that shaped our physiology.
One of the measurable consequences of modern life has been the steady rise in vitamin D deficiency across populations. At the same time, elevated cholesterol levels are increasingly observed—even in younger individuals. The connection is not mystical. It is biochemical.
When ultraviolet B radiation strikes the skin, it converts 7-dehydrocholesterol into pre-vitamin D3, which then becomes vitamin D3. That molecule is processed in the liver (25-OH D) and kidney (1,25-OH D) into an active hormone that influences immune function, bone metabolism, and systemic regulation. Without solar exposure, that pathway is largely silent.
This does not mean that sunlight alone will normalize severe hyperlipidemia. Cholesterol metabolism is influenced by diet, insulin signaling, inflammation, hepatic synthesis, and physical activity. But removing sunlight from the equation removes one of the evolutionary regulators of the system.
Modern humans live in a state of partial environmental deprivation.
Low vitamin D correlates with immune dysregulation and possibly with increased autoimmunity. Circadian disruption affects melatonin production, sleep architecture, glucose regulation, and hormonal balance. Sun exposure also triggers nitric oxide release in the skin, influencing vascular tone.
We have moved from one extreme to another.
At one time, survival depended directly on the Sun. Now we behave as if it were an optional accessory—or a threat.
The solution is not excess. It is balance.
Not hours of burning exposure.
Not reckless dismissal of dermatologic risk.
But measured, appropriate contact with the environmental force that shaped us.
Our bodies are resilient. We would not be here otherwise.
The question is whether we remember the conditions under which that resilience evolved.
