Why Not All Circadian Disruption Is the Same

We often talk about circadian disruption in broad strokes: jet lag, shift work, or chronic sleep loss. But what if not all circadian disruption is the same? What if the dim glow of a screen in the evening, something so small and so ordinary, sets off its own distinct biological cascade?

That question became the focus of my Master’s research, a project that helped me to get a research fellowship at the University of Oxford by the Sleep and Circadian Neuroscience Institute (SCNi) where I spent four months last year on site there working on. I was warmly welcomed by Professor Russell Foster and guided by Professor Stuart Peirson. 

From day one, it was immersive: tracking mouse activity late into the night, analyzing microbiome shifts, analysing RNA sequencing and learning from world-class researchers who live and breathe circadian science every day. It wasn’t just an academic project. It was an experience that reshaped how I think about light, biology, and health.

Looking back, those four months at Oxford weren’t just about running my study. Looking back, those four months at Oxford shaped not just my research. They deepened my understanding of how small environmental cues can ripple through the entire system.

The study we conducted explored the effects of dim light in the evening (DLE), just 4 hours of 20 lux light added after a normal 12-hour light cycle. A subtle shift. But what we found might change how we think about modern circadian stress.

Table of Contents:

1. It Started With a Question

2. Experimental Design and Methodology

3. What We Found and Why It Matters

   • Key Results from the Mouse Model

4. Why This Matters: The Mouse-Human Inversion

   • Takeaways for Your Daily Life

5. Not All Light Is Equal

6. A New Lens on Light Hygiene and Microbial Health

7. What Clinicians Can Do Now

8. Where We Go From Here

It Started With a Question

In clinical practice, I often meet patients who do everything right: they eat well, exercise, follow time-restricted eating, and optimize sleep. But something still feels off. Energy is inconsistent. Sleep is light and restless. Gut symptoms come and go.

I started wondering whether the issue wasn't the amount of light or food or sleep, but the timing of it all.

That led me to Oxford, where I designed a protocol to investigate whether chronic exposure to dim light in the evening, something so common in modern life, would shift biological rhythms enough to affect feeding behavior and gut microbiota.

Experimental Design and Methodology

To explore how dim evening light affects circadian biology, we conducted a 6-week study using male and female mice housed under two different lighting conditions: a standard 12:12 light-dark cycle (LD) and a dim light in the evening (DLE) protocol, where four hours of low-intensity light (20 lux) replaced part of the dark phase.

After a 3-week acclimatization period, mice were monitored for activity and sleep patterns using passive infrared sensors, while feeding behavior was analyzed using the SnackerTracker system. Fecal samples were collected at baseline and after DLE exposure for microbiome sequencing and functional pathway analysis. Despite no major differences in food intake or body weight, the DLE group showed clear shifts in behavior, microbial timing, and subtle changes in strain-level composition. All procedures adhered to UK animal research regulations, and multiple data streams were collected from the same cohort to reduce animal use.

What We Found and Why It Matters

Ever wondered how that late-night scroll on your phone might be messing with your gut? Our study on mice exposed to dim light in the evening (DLE) sheds light on the subtle ways artificial evening light disrupts circadian rhythms, sleep, feeding, and even the gut microbiome—without causing obvious metabolic crashes like weight gain.

Key Results from the Mouse Model

After 6 weeks of chronic DLE (4 hours at 20 lux), nocturnal mice showed clear signs of circadian misalignment, but with some unexpected twists:

• Delayed Activity Onset: A significant ~3.6-hour delay in when mice became active (ZT16:03 vs. ZT12:24 in controls; p < 0.001)

• Longer, Less Fragmented Sleep (bad news for mice): Total sleep time increased (15.8 ± 0.04 hours vs. 14.8 ± 0.07 hours in controls; p< 0.001), with fewer disruptions during both day (48.39 ± 0.8 vs. 67.32 ± 0.92; p < 0.01) and night (38.84 ± 0.66 vs. 49.61 ± 0.59; p < 0.01)

• Shifted Feeding Rhythms: Feeding peaks delayed by several hours, though overall food intake and body weight remained unchanged

• Modest Microbiome Boost: Gut bacterial alpha diversity slightly increased (Shannon index), while beta diversity stayed stable

• Family-Level Stability with a Twist: Decreased variance in their abundances under DLE of the Akkermansiaceae or Bifidobacteriaceae bacteria groups, suggesting tighter population control—potentially an adaptive response to misalignment

These findings build on prior work, like Tam et al. (2021), showing mild evening light realigns rhythms without total disruption.

Why This Matters: The Mouse-Human Inversion

Mice are nocturnal, so extra evening light extends their "day" and promotes more restful sleep. In humans (diurnal creatures), it's the opposite: evening light delays melatonin, fragments sleep, and shortens duration—often leading to poorer gut health over time.

• Sleep and Diversity Flip: In mice, more sleep correlated with higher microbial diversity. In humans, DLE-linked sleep issues are tied to decreased diversity and less resilient gut ecosystems.

• Feeding and Metabolism: Stable weight/intake in mice, but humans' late-night habits from DLE often derail metabolism, boosting risks for obesity or diabetes.

• Functional Over Compositional Shifts: The real story? Even without big microbiome makeovers, DLE rewired functions—like tRNA aminoacylation, oxidative stress responses, and B-vitamin metabolism. This hints at subtle "metabolic rewiring" from phase delays, influencing gut barrier integrity (via Akkermansia muciniphila) and immune modulation (via Bifidobacterium).

Clinically, it's a wake-up call: Small light tweaks can quietly alter gut rhythms, inflammation, and overall health. Future studies need time-resolved metagenomics to unmask strain-level changes—could Akkermansia or Bifidobacterium be early warning signs?

Takeaways for Your Daily Life

• Dim your screens after sunset to protect your rhythms

• Align meals with natural light cycles

• Prioritize sleep hygiene to support gut diversity

Our results underscore that in a world lit 24/7, timing is everything for health. Stay tuned for human translations!

Not All Light Is Equal

Emerging research shows that light intensity, wavelength, and timing each produce unique circadian effects. Blue-enriched light in the evening, for instance, more potently delays melatonin release and circadian phase than warmer tones. But even warm ambient light at common indoor levels can have substantial effects when mistimed.

A recent meta-analysis confirmed that evening light exposure, particularly in the hours before bed, consistently delays melatonin onset and alters sleep structure across multiple studies (Tähkämö L. et al., 2018). This suggests that even routine lighting habits can drive meaningful circadian misalignment, especially over time.

But what’s striking is how this disruption differs from extreme jet lag or total light deprivation. Instead of fully resetting the clock, it subtly shifts it, creating an internal mismatch between sleep-wake timing and the molecular rhythm within peripheral tissues.

A New Lens on Light Hygiene and Microbial Health

DLE didn’t cause dysbiosis, but it changed the timing of feeding, which is a key microbial cue. Most gut bacteria operate on feeding-based rhythms, not just light. When feeding shifts, microbial metabolite production shifts too.

In our mice, alpha diversity rose and functional pathways related to amino acid and oxidative stress metabolism were enriched, an adaptive temporal response rather than collapse. In humans, the inversion applies: when evening light delays sleep and compresses rest, diversity and resilience tend to fall over time (Tähkämö et al., 2018; Lotti et al., 2023).

The intestinal circadian clock directly regulates microbial composition and turnover through rhythmic epithelial signaling (Heddes M. et al., 2022). Disruption of this rhythm, either through genetic manipulation or environmental cues, leads to microbial imbalance and impaired gut barrier integrity.

Microbiota rhythms are sensitive not only to diet quality but also to the timing of meals, sleep patterns, and light exposure (Lotti S. et al., 2023). When these cues become misaligned, the microbial ecosystem may enter a less resilient, pro-inflammatory state, even in the absence of overt dysbiosis.

From a clinical perspective, this reinforces a central idea:

- It’s not just what we eat that matters. It’s when we eat, sleep, and see light.

What Clinicians Can Do Now

1. Ask about evening light

   Many patients overlook screen exposure before bed. Ask directly: "Do you use a phone or laptop at night? Do you sleep in a totally dark bedroom? "

2. Support microbial resilience

   Encourage prebiotic fibers (inulin, flaxseed, legumes) and polyphenols

   Consider resistant starches to promote butyrate and microbial signaling

3. Anchor food to light

   Recommend consistent meal times during the daylight hours

   Aim for earlier dinners on most nights (and avoid bright/blue-heavy light thereafter) to keep feeding and clock signals aligned

4. Track sleep and rhythm patterns

   Use wearables or sleep logs to detect timing delays

   Pay attention to sleep duration and fragmentation

5. Consider tools for circadian recalibration

   Melatonin (0.3–1 mg) short-term in cases of delayed sleep

   Morning light exposure routines to re-entrain the SCN

Where We Go From Here

This study doesn't claim DLE is harmless, only that its effects differ from more extreme models of circadian disruption. For humans, especially those with metabolic issues, gut conditions, or disrupted sleep, this kind of low-level but persistent misalignment may still carry risk.

We also know that artificial light in the home environment isn’t benign. In a controlled study, evening ambient lighting at levels commonly found in living rooms and bedrooms delayed melatonin secretion, shortened REM sleep, and altered sleep architecture in healthy adults (Cain S. et al., 2020).

What we need now are human studies examining:

• Long-term effects of screen light on metabolic biomarkers

• Shifts in gut microbiome composition and short-chain fatty acid (SCFA) production

• Interaction between light timing, feeding, and sleep

As artificial evening light becomes a permanent fixture of modern life, understanding its nuanced impact on the gut-brain axis is no longer optional; it is essential.

Let’s Keep Exploring Together

If this article resonates with your clinical experience or curiosity, please share it with colleagues. Subtle shifts in light and timing may be shaping your patients' biology in ways we’re only beginning to understand.

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References:

1. Adriano dos Santos (2025). Circadian Rhythms and the Gut Microbiome: Investigating the Impact of Dim Light in the Evening. A thesis submitted for the degree of Master of Sleep Medicine. Deputy Director, Sleep and Circadian Neuroscience Institute (SCNi). Nuffield Department of Clinical Neurosciences. University of Oxford.

2. Heddes M., Altaha B., Niu Y., Reitmeier S., Kleigrewe K., Haller D., Kiessling S. (2022). The intestinal clock drives the microbiome to maintain gastrointestinal homeostasis. Nature. https://doi.org/10.1038/s41467-022-33609-x

3. Lotti S., Dinu M., Colombini B., Amedei A., Sofi F. (2023). Circadian rhythms, gut microbiota, and diet: Possible implications for health. Nutrition, Metabolism and Cardiovascular Diseases. https://doi.org/10.1016/j.numecd.2023.05.009

4. Tähkämö L., Partonen T., Pesonen A-K. (2018). Systematic review of light exposure impact on human circadian rhythm. Chronobiology International. The Journal of Biological and Medical Rhythm Research. https://doi.org/10.1080/07420528.2018.1527773

5. Tam S., Brown L., Wilson T., Tir S., Fisk A., Pothecary C., van der Vinne V., Foster R., Vyazovskiy V., Bannerman D., Harrington M., Peirson S. (2021). Dim light in the evening causes coordinated realignment of circadian rhythms, sleep, and short-term memory. PNAS. https://doi.org/10.1073/pnas.2101591118

6. Cain S., McGlashan E., Vidafar P., Mustafovska J., Curran S., Wang X., Mohamed A., Kalavally V., Phillips A. (2020). Evening home lighting adversely impacts the circadian system and sleep. Nature. https://doi.org/10.1038/s41598-020-75622-4