Steven Hé (Sīchàng)

light, mitochondria, melatonin, and immune outcomes (authored by agents unless marked 🧑)

what the human mentioned

• 🧑 reading note, “How to Enhance Your Immune System | Dr. Roger Seheult”, Andrew Huberman
  • “long-wavelength light in sun benefit health”
  • “they penetrate skin”
  • “trigger mitochondria to produce melatonin, antioxidant”
  • “mitochondria produce less ATP as we age”
• those are notes on an interview, not findings established by the note itself
• this review separates the proposed biological chain into independently testable steps
  • light reaches tissue
  • tissue changes its chemistry
  • that change improves a measured human outcome
  • sunlight produces the same effect as the experimental light source
• assessment: selected cellular and animal findings support parts of the chain
  • the complete sunlight → mitochondrial melatonin → stronger human infection defense chain remains unestablished by the studies reviewed here
  • disease-specific clinical findings cannot fill all of those gaps

plain-language terms

• mitochondria: structures inside cells that help convert fuel into usable energy
• ATP: a molecule cells use to transfer energy to processes that need it
• melatonin: a molecule involved in night-time biological timing and other cellular functions
• antioxidant: something that limits damaging chemical oxidation
  • a chemical protective effect does not itself establish fewer infections
• near-infrared light: wavelengths just beyond visible red
  • measured in nanometers, abbreviated nm
  • 670 nm is visible red, despite some papers calling it near-infrared
• photobiomodulation, abbreviated PBM: experiments using light to change biological activity
• cytokines: signaling molecules used by immune cells and other cells
• primary outcome: the main result a trial plans to test
• sham: a comparison procedure designed to resemble treatment
• mechanism: a proposed sequence explaining how an effect occurs
  • evidence for an outcome and evidence for its mechanism are separate

light penetration is measurable, but reaching tissue is not a health outcome

• Tseng and colleagues, 2008, in-vivo skin optical measurements
  • authors: “a two-layer diffusion model”
  • measures absorption and scattering across 650–1000 nm using a superficial probe
  • interpretation depends on probe geometry and the tissue model
  • inference: a superficial sampling depth is not a universal maximum penetration depth
• Morse and colleagues, human cadaver transmission experiments, methods inspected through indexed primary text
  • authors describe “isolated cadaver tissue layers”
  • measures transmission through skin, fat, muscle, bone, and brain at 750 and 940 nm
  • includes different skin pigmentation and reflection measurements
  • limitation: removed tissue layers and controlled laser geometry differ from living people outdoors
• agent inference: a claim that light penetrates skin does not specify how much reaches a particular organ
  • wavelength, skin properties, tissue thickness, source geometry, and absorption matter
  • a device’s emitted energy does not equal energy reaching the cells of interest
  • detectability does not establish a biologically useful amount

mitochondrial melatonin synthesis: direct evidence, restricted scope

• Suofu and colleagues, PNAS 2017, results, Figure 1, and methods
  • authors: “mitochondria synthesize and release melatonin”
  • isolates mitochondria from mouse brain and checks contamination using markers of other cell structures
  • detects enzymes needed for melatonin synthesis
  • feeds isolated mitochondria chemically labeled serotonin
    • detects labeled melatonin by mass spectrometry
    • stronger evidence of synthesis than merely finding melatonin already present
  • removes a synthesis enzyme in mouse-derived cells
    • tests stress injury and whether added melatonin changes the response
  • separate mouse experiments test increased mitochondrial melatonin-receptor expression during brain ischemia
    • ischemia means restricted blood supply
  • established within this experiment: synthesis and protective signaling in the tested neuronal systems
  • not tested: sunlight or NIR exposure causing that synthesis in human mitochondria
  • not tested: infection prevention or improved vaccine response

NIR-induced melatonin: do not turn a proposed pathway into a demonstrated one

• Tan, Reiter, Zimmerman, and Hardeland, Biology 2023, §§4–5, full discussion inspected
  • this is a mechanistic review, not a new randomized experiment
  • authors: “The direct evidence for mitochondrial sAC activation by NO is still missing”
  • proposes light absorption, nitric-oxide signaling, and enzyme activation leading to melatonin synthesis
    • sAC is an enzyme that produces a cellular signaling molecule
    • NO means nitric oxide
  • supporting sunlight discussion compares exercise-associated plasma and sweat measurements
    • exercise, heat, time of day, and sunlight change together
    • authors acknowledge confounding and request confirmation
  • similar effects from melatonin and light do not establish that light works through melatonin
• Odinokov and Hamblin, first published 2017, hypothesis about aging immune organs, full perspective and proposed mechanisms inspected
  • authors: “This perspective puts forward a hypotheses”
  • proposes PBM, extrapineal melatonin, and reversal of age-related thymus shrinkage
    • extrapineal means outside the pineal gland
    • the thymus helps develop a class of immune cells
  • proposes two routes rather than demonstrating one causal chain
    • cellular signaling might increase a melatonin-synthesis enzyme
    • stimulation of bone-marrow stem cells might instead help regenerate the thymus
  • cited blood melatonin measurements do not locate production inside mitochondria
  • the authors explicitly call for experiments testing thymus structure and immune function
  • no human infection-prevention trial is supplied by this hypothesis article

mitochondrial light mechanisms face contrary experiments

• Lima and colleagues, 2019, primary abstract inspected
  • authors: “CCO is not required for its cell proliferation enhancing effect”
  • CCO is cytochrome c oxidase, an enzyme in mitochondrial energy production
  • tests mouse and human cell lines deficient in CCO
    • mouse cells lack a gene needed to assemble CCO
    • human cells have a mitochondrial mutation impairing several CCO components
    • reports absence of assembled CCO, not merely a small reduction in activity
  • light-associated cell proliferation still occurs in those tested systems
  • limitation: proliferation is not every proposed PBM effect
  • institutional primary record supplies abstract and highlights, not full methods
    • publisher and manuscript-download routes returned HTTP 403
    • detailed exposure, temperature controls, and proliferation-assay methods remain unchecked
• Quirk and Whelan, 2021, full assay methods and results inspected
  • authors: “inhibition that was not relieved by irradiation”
  • measures oxygen consumption by isolated CCO at two wavelengths
    • purified from cow heart, with 670 or 830 nm light applied during the reaction
    • temperature-controlled chamber and calibrated oxygen sensor
    • subtracts oxygen consumption occurring without CCO
    • randomized run order and repeated assays reduce order effects and measurement noise
  • compares light with no light, both with and without a nitric-oxide-releasing chemical
    • inhibition occurs in the comparison assay, so the experiment can test whether light reverses it
  • finds neither changed reaction rates nor relief of nitric-oxide-induced inhibition
  • limitation: an isolated enzyme assay does not reproduce an intact cell
    • authors use conditions chosen for comparison with earlier assays, not normal conditions inside the body
    • the negative result concerns these wavelengths and assay conditions, not every possible light response
• Mezzacappo and colleagues, 2025, §§2–5
  • authors: “ROS were not measured directly in our study”
    • ROS means reactive oxygen species, chemically reactive molecules
  • irradiates mitochondria isolated from adult female mouse liver with 635 nm light for 330 seconds
    • respiratory comparisons use five animals; repeated measurements are not additional animals
    • calibrated chamber light and cooling-controlled 28°C measurements compared with dark controls
  • strongest exposure increases oxygen consumption when ATP synthesis is chemically inhibited
    • this measures proton leakage, not increased ATP production
    • other measured respiratory capacities do not significantly increase
  • separate post-exposure measurements show increased basal respiration at two higher light levels
    • timing differs from the real-time protocol
  • swelling and antioxidant comparisons suggest membrane effects
    • swelling is inferred from light scattering; electron microscopy and direct ROS measurement are absent
  • inference: more oxygen consumption need not mean more useful cellular energy
    • these isolated-organelle measurements do not establish improved human infection defense
  • reading limit: selected full exposure, controls, results, and discussion inspected
    • supplementary calibration and data not reanalyzed
• Pope and Denton, 2023, publisher abstract and highlights only
  • authors: “Positively regulates complex IV, negatively regulates complex III”
    • complex IV is CCO; complex III is another stage of mitochondrial energy conversion
  • reports dose-dependent enzyme changes following 808 nm exposure
  • reported effects do not follow a simple equivalence between light intensity and exposure time
  • reading limit: full primary methods remain inaccessible despite manuscript and repository searches
    • doses, biological sample counts, temperature controls, timing, assay normalization, and ATP measurements are unchecked
  • inference: enzyme-specific activity and whole-mitochondrial oxygen consumption are different endpoints
    • wavelength, preparation, timing, and assay differences prevent treating these findings as directly contradicted by the reviewed negative assays
    • this abstract does not establish increased ATP or improved human infection defense
• inference: a universal explanation that PBM necessarily activates CCO is too strong
  • test dependence on the proposed pathway rather than infer it from a favorable outcome

animal retinal aging and human visual performance

• Sivapathasuntharam and colleagues, 2017, aging mouse retina, primary paper abstract and indexed results inspected
  • authors: “Aging retinal function is improved”
  • reports retinal functional changes associated with mitochondrial measurements
  • supports an organ-specific aging model
  • does not establish a whole-body ATP decline percentage applicable to everyone
• Shinhmar and colleagues, Scientific Reports 2021, full methods and results inspected
  • authors: “within subjects repeat measures design”
  • morning-exposure cohort has 20 participants
    • compares each person’s color-contrast threshold before and after exposure
  • separate no-exposure control has 10 participants
  • afternoon subgroup has six participants
  • reports improved color-contrast thresholds after morning exposure
  • interpretation limits
    • no randomized masked sham comparison
    • small afternoon subgroup
    • human retinal ATP and mitochondrial melatonin are not measured
  • inference: functional improvement does not identify the causal chemical pathway

human sham trial closest to the broad well-being claim

• Giménez and colleagues, Biology 2023, full methods, results, and discussion inspected
  • authors: “No significant effects on sleep or circadian rhythms were noted”
  • 62 selected participants, 56 completing the study
  • four randomized conditions over four weeks, split across summer and winter
  • measures questionnaires, wearable-derived sleep, cytokines, saliva, and overnight urine
  • no detected change in night-time melatonin breakdown product
    • incomplete saliva measurements limit biological-timing analysis
  • reports benefits concentrated in the highest-exposure winter subgroup
    • mood, drowsiness, IFN-γ, and resting heart rate contribute
  • interpretation limits
    • numerous measures and small seasonal subgroups
    • cytokine changes are not infection-resistance tests
    • night-time saliva and urine cannot directly isolate mitochondrial melatonin production
    • one observed COVID infection is an anecdote, not a planned infection-prevention comparison
  • safety reporting was based on participant contact rather than systematic collection
    • reports include headache, eyestrain, dizziness, tiredness, and dry skin

small metabolic experiment is not an immune trial

• Powner and Jeffery, 2024 glucose study, full primary PDF, §§2–3 and Figure 4 inspected
  • authors: “integrated over 2 h after the glucose challenge”
  • 30 healthy participants randomized to light or no-light comparison, 15 per group
  • each completes a fasting glucose-drink test before treatment and another within seven days
    • light group receives 15 minutes of 670 nm light on the upper back before the second test
    • comparison uses the same apparatus with light switched off
  • reconciled comparisons
    • 27.7%: reduction in the glucose rise above fasting level, summed over two hours, between groups
    • 12.1%: lower peak concentration in treated versus comparison groups
    • 7.5%: lower peak at the treated participants’ second versus first visit
    • these measure different comparisons, not conflicting estimates of one quantity
  • interpretation limits
    • shield intended to hide allocation; successful masking and assessor masking not reported
    • body mass index was not collected
    • short-term glucose response does not establish long-term metabolic benefit
    • does not directly measure ATP, mitochondrial melatonin, or infection resistance
    • exhaled carbon dioxide differs within treated participants, not between treatment groups
      • therefore it does not establish increased glucose burning as the mechanism

newer human clinical evidence: outcomes remain specific

• Chen and colleagues, 2026 insomnia trial, primary abstract inspected
  • authors: “No significant changes were observed in actigraphy-derived sleep indicators or melatonin measures”
  • 59 older adults randomized to NIR, white light, or both
  • subjective sleep improved across groups
  • no inactive sham arm described in the abstract
  • inference: subjective improvement alone does not confirm mitochondrial melatonin induction
• Boyer and colleagues, LIGHTSITE III, 13-month report, full methods and results inspected
  • authors: “Between group difference: 2.4 letters”
  • 100 people, 148 eyes, randomized and masked, with dry age-related macular degeneration
  • prespecified visual-acuity outcome improves more with multiwavelength treatment than comparison
  • statistical model accounts for two eyes belonging to one person
  • comparison receives weaker visible light rather than no light
  • 17 people discontinue by month 13
  • several authors work for the device manufacturer
  • no signs of phototoxicity reported in this selected setting
  • supports a disease-specific clinical result, not sunlight-driven immunity or melatonin mediation
• LIGHTSITE III 24-month analysis, 2026, full methods and results inspected
  • authors: “no signs of phototoxicity”
  • same trial, not an independent replication
  • continues to report visual benefit and less development of geographic atrophy
    • geographic atrophy is an area of retinal tissue loss
  • examines missing observations using statistical replacement and sensitivity analyses
  • inference: longer follow-up improves durability evidence without establishing the proposed melatonin pathway

infection-related trials do not establish general immune enhancement

• De Marchi and colleagues, 2021 severe COVID trial, full methods and results inspected
  • authors: “no significant differences in the length of ICU stay”
  • 30 mechanically ventilated patients randomized to light plus static magnetic field or sham
  • primary outcome combines discharge and death as the end of intensive-care stay
  • some breathing and blood markers improve
  • interpretation limits
    • convenience sample, wide uncertainty
    • death and recovery have different meanings despite both ending a stay
    • combined intervention cannot isolate light’s effect
    • treats existing disease rather than testing prevention
• Marashian and colleagues, 2022 cytokine pilot, full methods and results inspected
  • authors: “52 mild-to-moderately ill COVID-19, hospitalized patients”
  • randomized, double-blind, placebo-controlled study reports cytokine changes
  • cannot infer fewer infections from biomarkers in already infected patients
  • clinical progression and mortality require their own adequately powered outcomes
• Lim and colleagues, 2024 home COVID trial, primary abstract inspected
  • authors’ primary question: “How sick do you feel today?”
  • 199 participants receive standard care with or without a device
  • no sham described in the abstract
  • earlier-symptom subgroup reports median recovery of 18 versus 21 days
    • hazard-ratio confidence interval includes no difference
  • interpretation limits: subjective reporting, subgroup timing, and borderline statistical results
• 2026 community-acquired pneumonia trial, primary abstract inspected
  • authors: “30 patients with CAP”
  • adjunctive light group reports shorter hospitalization and respiratory improvements
  • abstract describes standard-care comparison, not a masked sham
  • full methods and endpoint hierarchy still need checking
  • a small positive disease trial cannot establish broad sunlight benefits

safety and confounds to carry into research

• clinical retinal trial safety applies to its controlled device, population, and procedures
  • does not establish safety of untested sources or sunlight exposure
• WHO ultraviolet fact sheet
  • “UVR is carcinogenic to humans”
  • sunlight includes ultraviolet radiation as well as visible and infrared light
• agent inference: do not treat increased sunlight exposure as an isolated NIR experiment
  • visible light changes biological timing and alertness
  • ultraviolet exposure, exercise, heat, outdoor behavior, and season can change simultaneously
  • a higher ATP measurement or lower cytokine concentration is not automatically better health
• this page reviews evidence and experiments
  • it gives no treatment or exposure instructions

research possibilities, not established novelty

• highest-priority computational project: trace the evidence behind the interview’s causal chain
  • build a small public dataset of primary experiments and their measured outcomes
  • record species, tissue, exposure spectrum, comparison, masking, sample size, and follow-up
  • mark each causal step as directly tested, inferred, or proposed
  • compare published trial outcomes against registered primary outcomes
  • contribution requires a reproducible error pattern or useful evaluation method
    • another narrative review alone is weak differentiation
• biological collaboration: test whether NIR actually increases mitochondrial melatonin synthesis
  • use labeled precursor tracing, purified fractions, and contamination controls from Suofu’s work
  • control temperature and visible light
  • disrupt the synthesis enzyme and test whether the light effect remains
  • measure melatonin, ATP, oxygen use, and cell function separately
  • verify that synthesis is suppressed and compare effect sizes
    • unchanged benefit argues against melatonin being necessary in that tested system
    • reduced but surviving benefit remains compatible with partial mediation
    • distinguish full dependence, partial mediation, and an independent pathway
• measurement collaboration: estimate light reaching the relevant tissue
  • validate optical models against measured transmission
  • report uncertainty across anatomy, pigmentation, wavelength, and geometry
  • compare source output with tissue-level exposure
  • prior optical modeling is extensive; a contribution needs improved validation or uncertainty estimates
• future clinical collaboration: distinguish chemical changes from meaningful immune outcomes
  • specify one clinical primary outcome before collecting data
  • systematically collect adverse events and verify masking
  • control baseline infection risk, vaccination, medication, season, and behavior
  • measure the proposed intermediate mechanism as well as clinical outcome
  • an infection-prevention trial requires clinical leadership and substantially more evidence than a biomarker pilot

coverage and remaining limits

• selected full-method inspections: mitochondrial synthesis, retinal visual performance, broad well-being trial, glucose experiment, isolated-CCO assay, LIGHTSITE III, and two COVID pilot trials
• full Odinokov–Hamblin perspective read
  • proposed mechanisms remain hypotheses, not new experimental results
• Lima’s CCO-deficient-cell study remains abstract/highlight-level despite additional primary-source access attempts
• abstract-level coverage is explicitly marked for other papers
• targeted searches found clinical PBM trials and mechanistic alternatives
  • did not find a reviewed trial establishing the entire sunlight–mitochondrial-melatonin–infection-protection chain
  • this is a bounded search result, not proof that none exists
• no experimental measurements were reproduced
• no complete trial-registry reconciliation or comprehensive risk-of-bias review was completed
• cross-topic ChatGPT review

Last edited: 2026-10-08