Most people have heard that light matters for health. But few understand how it works — and even fewer know about lux, the specific measure of light brightness that may be one of the most powerful and overlooked levers for your metabolism, weight, hormones, and sleep. In this episode, circadian health expert Sarah Kleiner goes deep on the biology behind lux, why infrared light is essential and largely absent from modern life, and what you can practically do about both — today.

About Sarah Kleiner

Sarah Kleiner is a circadian health coach, educator, and creator of the MyCircadian App, with over 15 years of experience working with clients all over the world. Board Certified in Applied Quantum Biology by the American Naturopathic Medical Board and certified by the Institute of Applied Quantum Biology, she is also certified in nutrition by the National Academy of Sports Medicine, completed Dr. Tom Cowan’s New Biology Clinic. Sarah specializes in helping clients overcome weight loss resistance, hormonal imbalances, and chronic health challenges through the principles of light and circadian biology. She hosts the top 2% podcast Evolving Wellness, writes a popular research-backed Substack, and can be found on Instagram at @sarahkleinerwellness.

What You’ll Learn

• Why lux — not just blue light — is the brightness signal your brain uses to determine what time of day it is, and why getting it wrong derails metabolism, hormones, and sleep
• The research-backed link between morning lux timing and BMI — including the 500 lux threshold that makes a measurable difference
• Why lux exposure is cumulative throughout the day, and how to track it with the MyCircadian App’s new lux session timer
• What infrared light does for mitochondrial function, blood sugar regulation, and eye health — and why modern environments have stripped it out almost entirely
• How to set up your evening light environment for real circadian benefit, including the lux threshold at night that matters more than most people realize
• What deuterium is, why it’s emerging as a metabolic marker, and why the circadian lifestyle may lower it independent of diet
• The fascinating research showing women stopped cycling with the moon around 2010 — and what that tells us about smartphones and circadian disruption

Episode Timestamps

• 00:00 — Introduction: Why we never stop talking about light
• 02:13 — Beyond blue light: What lux is and why it matters more than most people know
• 05:30 — How lux affects metabolism, mitochondria, and weight loss resistance
• 07:45 — The study: for every hour morning light is delayed, BMI increases by over one point
• 09:22 — Infrared light: what it does, why we’re not getting it, and what Dr. Glen Jeffery’s research reveals
• 11:16 — Mitochondria as biological sensors: how light environment instructs energy production
• 14:34 — The lux weight loss study: the group getting 10,000 lux lost nearly double the weight
• 16:39 — Lux is cumulative: how to use the MyCircadian App’s lux session timer
• 18:43 — Circadian amplitude: why getting lux throughout the day strengthens your entire rhythm
• 21:08 — Morning routines when it’s still dark: grounding, cold exposure, red light, and more
• 23:02 — Practical infrared tips: incandescent bulbs, open windows, and the desk lamp fix
• 28:49 — Deuterium as a metabolic marker: what it is and why the circadian lifestyle may matter more than diet
• 34:17 — How to measure your deuterium and what the numbers mean
• 38:37 — Evening lux: the threshold that matters, how long before bed, and how to measure it at eye level
• 44:22 — Blue blockers + screen brightness: why you need both, not just one
• 45:07 — The moon and women’s health: why cycles synced to the moon until smartphones arrived
• 48:56 — For practitioners: how to use the MyCircadian App with clients and the bulk practitioner program

Key Concepts Glossary

Lux — A measure of light intensity or brightness as perceived by the human eye. Distinct from blue light wavelength discussions, lux refers to the amount of light hitting a surface. The eye’s light-sensitive cells respond to both blue wavelength and lux, and continue signaling “daytime” to the brain for 2–3 hours after bright light exposure ends.

Circadian Amplitude — The strength or robustness of your circadian rhythm. A high-amplitude rhythm means strong, clear peaks and troughs in biological processes across the day. Low amplitude — caused by too little bright light in the day and too much at night — is associated with depression, metabolic disease, poor sleep, and increased mortality risk.

Infrared Light — Long-wavelength light (approximately 700–2500 nm) that is abundant in natural sunlight but almost entirely absent from LED lighting. Research by Professor Glen Jeffery at UCL shows infrared light is essential for mitochondrial ATP production, blood sugar regulation, and ocular health. Modern indoor environments are effectively “infrared starved.”

Deuterium — A naturally occurring heavy isotope of hydrogen found in water. When it accumulates in mitochondria, it slows energy production. Levels below 130 parts per million are associated with reduced risk of metabolic disease and cancer. Emerging evidence suggests circadian health, not just diet, plays a significant role in keeping levels low.

Zeitgeber — A German word meaning “time giver.” Refers to any environmental cue that synchronizes the body’s internal clock. Light is the primary zeitgeber; food timing, temperature, and exercise are secondary ones.

Resources & References

Tools & Apps

• MyCircadian App: mycircadianapp.com
• Iris screen software: iristech.co

Research Referenced

• Morning light timing and BMI — Reid et al., Northwestern University, PLOS ONE (2014): journals.plos.org | Plain-language summary: Northwestern News
• Dr. Glen Jeffery on infrared light and blood glucose — Powner & Jeffery, Journal of Biophotonics (2024): onlinelibrary.wiley.com | UCL News overview: ucl.ac.uk
• Dr. Sean Cain on daytime light, mood, sleep & circadian outcomes — Burns, Cain et al. (2021): pmc.ncbi.nlm.nih.gov
• Brighter nights, darker days and mortality risk — Windred, Cain et al., PNAS (2024): pnas.org

People Mentioned

• Prof. Glen Jeffery — UCL Institute of Ophthalmology
• Dr. Sean Cain — Monash University / Flinders University
• Scott Zimmerman — infrared light researcher, collaborator with Glen Jeffery
• Dr. Luc Boros — deuterium researcher
• Dr. Andrew Huberman — circadian and neuroscience educator

For Practitioners

Everything Sarah covers in this episode — lux thresholds, infrared deficiency, circadian timing for metabolic health — is the kind of foundational framework that transforms clinical outcomes when applied systematically. The practitioners seeing the greatest results are those who treat light environment as a primary variable, not an afterthought.

If this episode resonated, the IAQB Certification in Applied Quantum Biology is where clinicians go to apply this work at a clinical level — building the depth to integrate light, circadian biology, and bioenergetics into practice. Learn more here.

For Everyone

If this episode got you thinking differently about your light environment, you don’t have to navigate it alone. The Quantum Biology Collective Free Community is where curious people and health-conscious individuals gather to learn, ask questions, and connect around exactly these topics — light, circadian biology, mitochondrial health, and the emerging science that mainstream medicine hasn’t caught up to yet.

Inside the community you’ll also find the QBC Practitioner Directory — a searchable list of light-informed clinicians and coaches who are already applying these principles in their practice. If you’re looking for a practitioner who understands lux, infrared, and circadian health as foundational — not optional — this is where to find them.

👉 Join the free QBC community and access the practitioner directory here

Transcript Excerpt

Meredith: For someone midlife who wants to lose weight — they’ve been doing the things that worked in their twenties and thirties and it’s just not working anymore — where does light come in?

Sarah: If your light environment is incorrect, if your body thinks it is dawn or dusk all day long because you never go outside, your metabolism and your mitochondria go into a bit more of a conservation mode. Your mitochondria are always looking for signals. They’re kind of like these biological sensors that we’ve got all over the body. If they don’t get the right amount of brightness in the morning and through the day, they will down-regulate certain functions. Thyroid can down-regulate.

Meredith: So we can be doing all the right things with food —

Sarah: And there was a study I post about — for every hour that morning light was delayed, BMI increased by one point. The threshold was about 500 lux. Indoors, your lux is going to max out at about 100 to 300. Outside, even on a cloudy day, you’re going to get 10,000.

Meredith: And that’s just brightness — not even blue light specifically.

Sarah: Exactly. Everyone talks about blue light. Nobody talks about lux. And these cells in our eyes that tell our brain what time it is — they are sensitive to both. And they continue signaling for two to three hours sometimes. So if you turn the lights off at ten o’clock at night, those cells may take two to three hours to stop signaling to the brain that it’s daytime.

Meredith: So “don’t look at screens before bed” is just the beginning of the conversation.

Sarah: It really is. Lux and BMI are heavily correlated. Lux and mental health. The more we have these principles of light dialed in, the less of an issue some of these midlife things become. This is foundational. If this is not in place, all the protocols you pile on may not work as well — if at all.

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