1. UV‑B →
Vitamin D
→ calcium, muscle & metabolic vitality
- Photon event: 290‑315 nm UV‑B splits the B‑ring of 7‑dehydrocholesterol in the epidermis, forming previtamin D₃, which thermally isomerises to vitamin D₃ (cholecalciferol).
- Hormone cascade: In liver (CYP2R1) and kidney (CYP27B1) it becomes calcitriol, a steroid hormone that:
- up‑regulates intestinal Ca²⁺ transport and bone mineralisation,
- modulates 1 000+ genes linked to mitochondrial oxidative phosphorylation and insulin sensitivity,
- fine‑tunes both innate and adaptive immunity (macrophage activation, T‑cell balance).
- Net energy win: Better calcium handling supports ATP‑hungry muscle contractions; immunomodulation reduces the “silent energy tax” of chronic inflammation.
2. UVA →
Nitric‑Oxide “flash”
→ vascular energy savings
- Photon event: 315‑400 nm UVA liberates NO stores (nitrate/nitrite) in the dermis.
- Physiological payoff: Rapid NO‑mediated vasodilation lowers peripheral resistance and blood pressure. Your heart does the same circulatory work with fewer beats—an efficiency dividend.
3. Visible blue‑cyan light →
Circadian clock
→ metabolic timing
- Photon event: 460‑480 nm light excites melanopsin in intrinsically photosensitive retinal ganglion cells (ipRGCs).
- Neural route: Signals reach the suprachiasmatic nucleus (SCN), the body’s “master pacemaker.”
- Down‑stream effects:
- Suppresses nocturnal melatonin and synchronises cortisol, growth hormone, and glucose rhythms.
- Aligns mitochondrial peak performance with daytime food intake, boosting daytime alertness and night‑time repair. Disordered light cues (night shifts, perpetual indoor life) uncouple these cycles and drain energy.
4. Bright daylight →
Serotonin & mood circuits
→ felt energy
- Retinal and possibly cutaneous phototransduction elevate central serotonin, lifting mood and motivation—the subjective side of “having energy.”
5. Red/Near‑Infrared (600‑1 000 nm) →
Photobiomodulation
→ mitochondrial turbo‑boost
- Photon event: Deep‑penetrating red/NIR photons are absorbed by chromophores in cytochrome‑c oxidase.
- Cellular response: Electron transport speeds up, proton gradients steepen, and ATP synthesis rises while ROS signals trigger repair pathways. Human trials report better muscle recovery, lower blood glucose, and neuro‑protective effects.
6.
Indirect solar calories
—plants do the hard part
Every carbohydrate, fat, or protein molecule you eat was once assembled in a chloroplast with solar photons. The mitochondria in your cells simply “withdraw” that deposited energy. So even your breakfast is bottled sunshine!
Why evolution wired us this way
| Sunlight Function | Evolutionary Advantage |
| Vitamin D synthesis | Enabled terrestrial vertebrates to maintain calcium balance without dietary limitation. |
| Circadian entrainment | Anticipatory regulation of metabolism → energy efficiency, predator avoidance. |
| NO release | Rapid thermoregulation and cardiovascular adaptation without extra metabolic cost. |
| Photobiomodulation | Opportunistic boost to cellular repair in sun‑rich environments. |
Practical, sun‑safe tips to
harvest the light
(without getting burned)
- Morning light (10–30 min before 10 a.m.) – strongest circadian anchor, minimal UV risk.
- Short, frequent midday bursts (5–10 min skin exposure) – tops up vitamin D; adjust for latitude, season, and skin tone.
- Protect, don’t banish: SPF on face & neck after your minimal‑erythemal dose; hat + sunglasses preserve ocular and dermal health while still letting arms/legs soak UV‑B briefly.
- Red‑light indoors on dark days – clinically dosed LED panels (630/850 nm) can mimic natural NIR benefits.
- Keep windows open or use full‑spectrum bulbs if you are house‑bound—glass blocks UV‑B but visible/near‑IR still entrain your SCN.
The big picture
Sunlight is not just warmth on your skin—it is a multispectral signal that charges your bones, tunes your body clock, dilates your vessels, boosts your mitochondria, and brightens your mind. Handle it with respect and you carry a little stellar fire inside you all day long.