Dr Martin Moore-Ede is a former Harvard Medical School professor, leading circadian clock researcher, and Chief Health Advisor at LightHealth. He discovered the suprachiasmatic nucleus, the biological clock in the human brain, and has spent over 40 years studying how light governs human health.
Sunlight makes us feel better. Most people accept this instinctively. But the scientific explanation for why has, for decades, been reduced to a single nutrient: vitamin D. Sunlight triggers vitamin D synthesis in the skin. Vitamin D deficiency is associated with poor health. Therefore, sunlight is good for you because of vitamin D.
This is true as far as it goes. But it is a significant underestimate of what sunlight actually does to the human body.
The vitamin D explanation has a gap
If sunlight's health benefits were primarily delivered through vitamin D, then vitamin D supplements should replicate those benefits. The research on this has been disappointing. Large randomised controlled trials of vitamin D supplementation have generally failed to show the cardiovascular, cancer-protective, or longevity effects that sunlight exposure consistently delivers in epidemiological studies. For the long-term health evidence, see our piece on could more sunlight help you live longer.
Something else is going on.
Sunlight works through at least four distinct mechanisms
First, and most importantly for circadian health, sunlight entrains the biological clock. Morning exposure to bright, blue-rich natural light, hitting the specialised melanopsin photoreceptors in the retina, resets the circadian clock to the correct time, synchronising every organ and tissue in the body to a shared 24-hour rhythm. This synchronisation is foundational to metabolic health, immune function, hormone regulation, and cognitive performance.
Second, sunlight stimulates nitric oxide release through the skin. Nitric oxide is a potent vasodilator with immediate cardiovascular effects. Studies have shown that UV exposure from sunlight reduces blood pressure through this pathway, a benefit entirely independent of vitamin D.
Third, sunlight activates serotonin production via the skin and the visual system. Serotonin is the precursor to melatonin, meaning that good daylight exposure sets up good nocturnal melatonin production. It also directly affects mood, motivation, and resilience.
Fourth, there is emerging evidence that near-infrared wavelengths in natural sunlight, wavelengths almost entirely absent from indoor lighting, play a role in mitochondrial function and cellular energy production. Modern LED lighting has also stripped out other wavelengths — see our piece on the hidden health risk in your LED bulbs for what the absence of violet light is doing to eye health.
We have engineered sunlight out of our lives
We now spend approximately 90% of our waking hours indoors. The light inside our buildings is a pale imitation of natural daylight, lower in intensity by one to two orders of magnitude, shifted in spectrum, and completely static across the day. It does not rise and fall with the sun. It does not shift from cool blue-rich light in the morning to warm amber in the evening. It simply stays on.
The biological consequence is that billions of people are running a chronic mild circadian deficit, cut off from the full spectrum of signals that sunlight has been providing to human biology for hundreds of thousands of years.
What this means in practice
Getting outside in the first hour after waking is one of the highest-leverage health habits available. Fifteen to thirty minutes of morning sunlight, on the eyes and not through glass, which filters critical wavelengths, has measurable effects on circadian entrainment, sleep quality, and daytime alertness. No supplement replicates this. For what optimal light conditions actually look like, see our piece on why the summer solstice is the healthiest day of the year.
Where natural light is limited, in winter months, office-bound work patterns, or at high latitudes, spectrally tuned artificial lighting that delivers the right wavelengths at the right intensity can partially compensate. The emphasis is on partial. The goal is to close the gap, not pretend it does not exist.
