We often blame glucose swings on too many carbs, too little movement, or not enough sleep. 

But the missing variable for better metabolism might be the light we live and work under all day.

When working in an office under natural daylight (as opposed to static, artificial office lighting), adults spent nearly 7% more time in a normal glucose range.

At the same time, natural daylight nudged metabolism toward greater daytime fat oxidation (less reliance on carbs), especially around midday. 

And in muscle tissue (the body’s main glucose sink) circadian clock markers were elevated and shifted earlier in the day.

What makes this new study so interesting is that a single variable shift, the light environment while we're working, was enough to measurably change glucose profiles and daytime fuel utilization in less than a week. No diet change, no extra workouts, and no shifting bedtimes.

Is light a cure for diabetes? No. But today's newsletter will have you looking at light a bit differently. Not just as something we see, but as a "metabolic input" you can use to your advantage.

For the study, which was published in Cell Metabolism, 13 men and women with type 2 diabetes lived in a dedicated research facility for 5 days on two separate occasions. Each condition was structured like a typical workweek (Monday morning through Friday afternoon), with simulated "office hours" from 8 AM to 5 PM each day. 

During those hours, participants experienced one of two different light environments:

• A natural daylight environment where participants sat in front of wide windows facing outdoors, receiving broad-spectrum and naturally dynamic light across the day.

• An artificial lighting environment where participants sat in the same type of room, but separated by a lightproof barrier and illuminated by mixed fluorescent bulbs set to ~300 lux at eye level (static and "typical office" light conditions).

A key detail here is that the study didn't just compare "bright" to "dim." The researchers quantified the light stimulus and intensity using the traditional measure of lux (how bright light appears to our eyes) as well as melanopic EDI, a circadian-relevant metric that reflects how strongly light stimulates pathways in our eye's light-sensing cells (melanopsin/ipRGCs) that are responsible for communicating circadian signals to the brain. 

This ensured that not only was the natural daylight condition brighter, but it also delivered a much stronger biologically active daytime circadian signal with a natural variation that static indoor lighting just can't replicate.

The usual but important confounders were controlled for aggressively: light was standardized outside of office hours using blue-light-blocking glasses to prevent "accidental" daylight exposure (e.g., when using the bathroom); meal composition and timing were fixed; and physical activity was regulated to just 30 minutes of light-intensity stepping three times per day after meals.

This level of control is rare, but it's what makes the results (discussed next) feel so clean.

Study design and light exposures.

Natural daylight improves glucose control

Across the 5-day study, average glucose (measured via continuous glucose monitoring or CGM) wasn't different between natural and artificial light. 

However, under natural light, participants spent 51% of the time in a normal glucose range (80–130 mg/dL) compared to just 43% of the time under artificial lighting conditions, without a difference in time with low blood sugar (hypoglycemia) or high blood sugar (hyperglycemia).

That's 7.6 percentage points more time in a normal glucose range over just 5 days, without changing calories, activity, or sleep. (Note: the same pattern was sown when using the more common but liberal "time in range" threshold of 70–180 mg/dL set by the American Diabetes Association; 83% under natural light versus 77% under artificial light).

So, natural daylight exposure improved the quality of participants' glucose profiles and nudged them into a more favorable distribution throughout the day, in line with what's seen after structured exercise interventions in people with type 2 diabetes.

Glucose outcomes under natural and artificial light conditions.

Fat metabolism rises under natural light

The second major finding showed up specifically during daytime waking hours during the fourth day of the study, when participants were burning more fat and burning fewer carbohydrates at rest. The effect was especially apparent around midday and didn't show up overnight, when the two conditions looked similar in regard to substrate oxidation. 

Once again, this result is somewhat striking given that even with the same meals and the same activity, natural light led to less reliance on carbs and more fat burning. On the final day of the experiment when the participants consumed a "milkshake" while having their metabolic and hormonal response measured for several hours after, the post-meal metabolic response indicated slightly higher fat oxidation and lower carbohydrate oxidation in the natural daylight condition with one wrinkle: post-meal glucose was actually higher under natural light. 

The authors couldn't fully explain this, but raise the possibility that bright, circadian-active light may have time-of-day dependent effects on post-meal glucose. Since the test was done in the morning, we don't know if the response would look different if the same meal were tested later in the day.

Carbohydrate and fat oxidation under natural daylight and artificial light conditions.

A stronger nighttime melatonin signal

No light study is complete without melatonin, and this study measured two aspects of the "sleep hormone":

• Dim light melatonin onset or DLMO: this is when melatonin first begins to rise in the evening.

• Melatonin exposure after onset: this is the total melatonin exposure (technically the area under the curve) after DLMO, measured in this study from 9 to 11 PM.

Surprisingly, DLMO didn't differ between conditions, occurring at ~8:35 PM under natural light and ~8:42 PM under artificial light.

But total melatonin exposure was 16% higher under natural daylight compared to artificial light—the nocturnal circadian signal was stronger despite the clock not being shifted earlier or later.

Natural light shifts circadian clocks in muscle

One of the most compelling parts of the paper is that it asked where light might be acting downstream of glucose and whole-body metabolism.

They answered this question by measuring skeletal muscle circadian clock markers, and found that under natural light, the core clock genes Per1 and Cry1 increased in muscle tissue. And when researchers assessed circadian rhythms in participants' muscle cells, they found their rhythms were shifted about 45 minutes earlier (what's known as a phase advance).

Metabolomics also revealed a few interesting shifts in markers that could contribute to acute and lasting metabolic changes.

• Higher ether-linked phosphatidylethanolamine (PE-O)

• A tendency toward lower glucosyceramides (GlcCer)

• Broader patterns suggesting an association between natural light exposure and lower levels of other ceramides.

This matters because ceramides are often discussed as lipotoxic signaling lipids linked to insulin resistance; however, these were secondary and association-level outcomes, so we should interpret them as supportive here rather than definitive.

Melatonin levels and clock gene expression under natural and artificial light conditions.

What about sleep?

If there's a result that surprised me the most, it's that the natural light condition didn't improve subjective sleep quality (or mood); participants reported sleeping similarly under both natural and artificial light. There are several reasons why this might be the case: sleep quality was measured subectively and not obectively using trackers; the intervention lasted just 5 days; and participants were kept in a highly controlled facility with fairly structured bed and wake timing—there may not have been much room for sleep to improve in the first place.

But the lack of a sleep difference is also intriguing because it suggests that light can influence our metabolism through pathways that aren't related to better sleep. Circadian signals affect metabolism during waking hours too, via hormones, autonomic nervous system function, and clocks in our skeletal muscles and other tissues.

So the broader biology still holds up. Light, sleep, and metabolism are tightly intertwined, and in the real world (where our schedules, screens, and bedtime are far less controlled), the sleep pathway likely plays an even bigger role than it did in this lab set up.

Final thoughts

If you want a practical takeaway you can use tomorrow without overinterpreting the science, it's to treat daytime light as a metabolic input.

If you can, work near a window. Get outside during the first half of the day (even briefly… it's also a good opportunity to grab an "exercise snack"). 

And perhaps most of all, be sure to get some sunlight in your eyes in the morning before you step into an office or begin work for the day.

Then protect the other side of your day by keeping evenings dim. By doing this, you're helping your sleep and giving your metabolism a clearer signal for when it’s daytime and when it's not.

Ultimately, that will help you maximize all aspects of your health, productivity, and mood.

Consider Supporting as a FoundMyFitness Premium Member

Premium members get exclusive access to live AMA sessions and the full-length recordings once they're released, plus exclusive access to all episodes of The Aliquot. Here are a few hand-picked Aliquot episodes and member Q&As where I discuss the importance of light, circadian rhythms, and sleep for metabolic and mental health!

• Aliquot #121: Harnessing Natural Light for Better Sleep: I discuss how light is a potent regulator of our body's internal clock, driving critical processes like hormone release and sleep-wake cycles.

• Aliquot #5: Light exposure: Striking a Balance for Sleep and Mental Health: This Aliquot features Dr. Matthew Walker, a professor of neuroscience and psychology; and Dr. Charles Raison, a professor of psychiatry, to discuss why bright light exposure in the morning may be essential for wakefulness and a good mood.

• Q&A #46 with Dr. Rhonda Patrick (4/1/2023): How to start your circadian rhythm earlier and extend the overnight fasting period.

• Q&A #42 with Dr. Rhonda Patrick (12/3/22): Does eating one meal a day affect the microbiome and circadian rhythms?

You can also check out our Circadian Rhythm collection, which includes links to every piece of content across FoundMyFitness.

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Warm regards

— Rhonda and the FoundMyFitness team