Why your "carb tolerance" isn't just about food...

Oct 26, 2025

This innocent habit drops your insulin sensitivity up to 25 percent (makes you less carb tolerant 👀⁉️)

The Hidden Carb-Tolerance Killer

Everyone’s fighting about carbs - macros & diets, but here’s what almost no one’s talking about:

You can eat "perfectly" - and still wake up more insulin resistant - just because of the light in your room while you slept.

In a 2022 PNAS study, participants who slept under 100 lux of indoor light - roughly the glow of a TV, a bedside lamp or your neighbor's flood light shining into your bedroom - saw a ~25% reduction in insulin sensitivity the very next morning.

Today I want to dive a little deeper into this study & give you easy, inexpensive tools to help you mitigate your sleeping environment!

Keep reading for the article....

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Why Your Carb Tolerance Isn’t Just About Food

In 2022, researchers at Northwestern University published a study in PNAS called “Light Exposure During Sleep Impairs Cardiometabolic Function.”

Participants spent one night sleeping in a dark room (< 3 lux), then one night under 100 lux - roughly the glow of a TV or a bedside lamp.

(You can measure your lux with the lux meter inside MyCircadianApp - click here to download & use code Sarahk)

After that single night:

Insulin sensitivity dropped 20 - 25 %
Heart rate stayed elevated (the body thought it was still day)
Melatonin -the hormone that coordinates leptin, insulin and thyroid -was suppressed

Why does 100 lux impair your metabolism?

100 lux suppresses your melatonin!

Melatonin actually coordinates energy use and storage. When melatonin rises at night, it tells your mitochondria to switch from “fuel-burning” to “repair mode.”

If that signal is blocked by light exposure after dark your body stays in day mode, and several other hormones are impacted:

Insulin sensitivity drops (your cells can’t clear glucose efficiently)
Leptin signaling weakens (your brain stops registering satiety & loses touch with how much stored energy is available)
Cortisol stays elevated (fuel remains trapped in the bloodstream)
Thyroid light at night disrupts the hypothalamic-pituitary-thyroid axis, lowering thyroid output and slowing metabolic rate.
Estrogen: Light at night removes your natural brake on estrogen signaling, allowing excess estrogen activity that’s been linked to faster cell proliferation and higher tumor risk.

Food Still Matters - A Lot (let's not lose the plot)

Light sets your circadian rhythm and programs your mitochondria, but food also programs your metabolic rhythm & is a strong signal for your hormones.

Most people are underexposed to sunlight, overexposed to artificial light, and eating completely out of sync with their biology.

Light and food work together, and when you combine light hygiene with smart nutrition & timing, you restore leptin sensitivity, improve carb tolerance, and rebuild your metabolism at the mitochondrial level.

Your Circadian Sleep Environment Matters

If you can’t get your room completely dark, try this:

Blackout curtains or blackout shades - block street and neighbor light.
Eye mask - great travel or backup option (or if you have a spouse that doesn't understand your light habits).
Red night light - for bathroom trips or kids’ rooms. Red light has minimal melatonin impact.
Unplug or cover LED indicators (router, alarm clock, chargers).
Cool the room and go device-free 1-2 hours before bed. This lets melatonin and leptin rise together.
Measure your Lux with MyCircadianApp if in doubt (I like lux 10 and below the 3 hours leading up to bed & less than 1 in your sleeping environment).

Even these small changes can help re-sensitize your body to its natural carb and hormone rhythms.

Click here to read my most recent article on syncing your master hormones & gently supporting leptin this fall and winter.

The Next Step - Rebuild Leptin Signaling

If you’ve struggled with cravings, stubborn weight, or energy crashes: looking at light AND food is essential for the best - most sustainable results.

That’s exactly what we do inside the Leptin Reset - a 21-day protocol to realign light, food, and timing so your body burns efficiently again.

And for practitioners or deep-divers, the Leptin Master Plan breaks down the mechanisms between melatonin, leptin, thyroid, and mitochondria so you can apply this science to real clients and cases.