Most people think their bedroom is dark enough at night for a good night’s sleep, but the metabolic data suggests they are not dark enough..
William Wallace, Ph.D. explains how the threshold where measurable harmful effects show up is well lower than most standard sleep environments, so we would do well to consider how to truly darken our bedrooms.
Here’s why..
Mason and colleagues ran a controlled laboratory study at Northwestern University. Twenty healthy adults slept for one (1) night under one of two conditions:
1. a very dimly lit room (less than 3 lux, effectively dark) or
2. a moderately lit room (100 lux, roughly the brightness of a hallway nightlight or small, low watt lamp left on).
The 100 lux night produced measurable changes the next morning:
• Insulin resistance was higher.
• Nighttime heart rate was elevated.
• Heart rate variability was reduced, indicating sympathetic nervous system activation (the body’s “fight or flight” response during stressful situations).
Just one (1) night of moderate ambient light during sleep was enough to push cardio-metabolic markers in an unfavorable direction.
The follow-up question is whether that single-night signal translates into long-term disease risk.
Obayashi and colleagues, had already provided population-level data that 5 lux of bedroom light tripled diabetes incidence over 42 months. Their cohort of 678 elderly Japanese adults without diabetes at the beginning of the study had bedroom light intensity measured over consecutive nights. The 128 participants whose bedrooms averaged 5 lux or more had a higher incidence rate compared with the 550 participants whose bedrooms averaged below 5 lux.
To put 5 lux in context:
• Direct sunlight measures around 50,000 lux.
• A bright office is around 500 lux.
• A living room in the evening is around 50 lux.
• A hallway nightlight is around 10 lux.
• Five lux is roughly what reaches your bedroom from a streetlight through closed curtains, or from an LED display across the room, or from a phone face-up on the nightstand. It is well below the threshold of “feels dark” to most people.
The mechanism that Mason’s data points to is sympathetic activation. Even modest ambient light during sleep keeps the sympathetic nervous system more engaged, which manifests as higher heart rate, lower heart rate variability, and impaired insulin sensitivity the next morning.
Repeated chronically across years, this is a plausible pathway to the diabetes signal Obayashi observed at the population level.
Three caveats are worth being explicit about.
First, Mason 2022 is a small acute study (one night). The signal is robust, but extrapolating to chronic effects requires interpretation.
Second, Obayashi 2020 is observational cohort data. The 3.74x incidence rate ratio (seen in the graph image) is an association, not proof of causation, and despite adjustment for known confounders, residual confounding is always possible in observational designs.
Third, the Obayashi cohort was specifically elderly Japanese adults (average age 70.6). Generalization to younger populations and other ethnicities is uncertain, though the underlying circadian and sympathetic biology is known to occur across humans.
What this means in practice… The lever is mechanical, not pharmacological.
1. Pull the curtain.
2. Cover the LED clock.
3. Move the phone out of the room (preferable). [It is an EMF risk, and I give workable suggestions for all EMF risks in the bedroom here.]
4. Tape over standby lights on electronics.
The cost is minutes of effort and no money.
→ The Mason randomized controlled trial (RCT) shows the next-morning warning signal from a single moderately lit night.
→ The Obayashi cohort shows the population-level diabetes association.
Together they are not proof, but they are aligned, and the practical lever to change the problem is cheap.
Is Your “Dark” Bedroom Dark Enough? The Bottom Line
Below 5 lux the epidemiological warning signal disappears, while most bedrooms with any electronic devices, any uncovered windows, or any hallway light bleed are well above it. If you wake up and can see across the bedroom before turning anything on, you are probably above the threshold.
Excerpts from William Wallace, Ph.D.
***I want to add that women wanting to conceive need to manage their light exposure during sleep. When exposed to light at night, our melatonin levels drop and our estrogen levels rise. This can disrupt hormone balance, resulting in ovulation problems, menstrual irregularities, and poor egg quality. Studies have shown that women who work night shifts or are exposed to a lot of light at night have lower fertility rates and a higher risk of miscarriage and ovarian cancer. (source)
You can purchase a lux meter from most hardware stores or online retailers. These devices are simple to use and provide accurate readings of luminance levels.
“In peace I will lie down and sleep, for you alone, Lord, make me dwell in safety.” ~Psalm 4:8
***For the Full Spike Protein Protocol to protect from transmission from the “V” and to help those who took the “V”, go here.
***If you found value in this writing, please share it, discuss it, and subscribe to my FREE newsletter. Independent, ad-free work like this spreads because of readers like you.
Also, please consider supporting my work by using my Amazon affiliate link when purchasing from there.
Censorship is real, so my Pinterest account was suspended; thankfully, a big part of my main board is still alive through this link!
You can also find me on Facebook, Gab, MeWe, X (Twitter), and Instagram.
Medical Disclaimer: I am no longer a practicing medical professional, and I am not doctor. I am a mother. I do seek scientific confirmation of the safety and effectiveness of the herbs and remedies I use. Using remedies is a personal decision. Nothing I say on this blog is intended to treat or prevent disease. Consult your own doctor.©2026 Deep Roots at Home • All Rights Reserved
Katie
Wow, this is very interesting re: such low amounts of light causing trouble. We’ve worked at sleep hygiene for a few years and use a red bulb in a bathroom nightlight. Our alarm clock also has a red display.
My struggle re: sleep is that I sleep better with fresh air coming through the window, but my black-out blinds have to be up a couple inches to let the fresh air in, then a little light comes in, too. There are no street lights out in the country where we live, but the town lights (LED?) reflect off the clouds.
I’ve read that ladies can allow a little moonlight into their bedrooms at night for three nights over the full moon to regulate their cycle.
Getting 15-30 minutes of early morning light daily (no glasses/sunglasses) is supposed to really help regulate circadian rhythm over several weeks/months. I wonder if getting morning daylight would help folks improve sleep, even if their rooms aren’t dark enough at night?
Katie
Bea Ann Ann Bridges
Hello Katie, I’m replying about the last question in your comments. I asked some folks I know of who have researched this topic extensively and have a website that carries items to benefit circadian rhythms and melatonin production. Following is her answer.
” Thank you for sending this over! I love seeing more people talk about light and circadian health.
For that reader’s question, the way I would think about it is this: morning light and nighttime darkness are both important, but they do different jobs. Morning light helps anchor the body’s daytime rhythm, while darkness at night helps protect the sleep signal. So yes, getting outside for morning light is a wonderful habit, but I wouldn’t think of it as fully “canceling out” light exposure at night.
This Science of Sunrise video would be a great place to start on the morning-light side of the question:
https://www.youtube.com/watch?v=Cfl4ZZGzQOo
And for deeper questions like this & Tools, we also do a monthly Q&A on our YouTube channel. That is the best place to leave questions like this in the comments so they can be considered for the next Q&A:
https://www.youtube.com/@Dreamwalkerzai“
Bea Ann Ann Bridges
Also wanted to post these links to a few presentations they’ve done that are so informative. I’m listening to one of them now and learning a lot about light, circadian rhythms and melatonin that I’ve never known and it’s fascinating!
Light Hacking | Thaddeus Owen | TEDxGullLake
https://www.youtube.com/watch?v=7_hX7d2NmNE
The Lie of 9 to 5: Fixing Your Circadian Rhythm | Thaddeus Owen
https://www.youtube.com/watch?v=VujtgcVw2vY
Light, Sleep and Longevity Reclaiming Your Circadian Rhythm in a Modern World — Thaddeus Owen – Weston A. Price Foundation
https://www.youtube.com/watch?v=piv8fU0CQYQ
Katie
Thank you very much, Bea Ann!
I’ll do my best to watch those in the coming weeks. From what I’m reading in the health world lately, getting the right light, magnetism, and water are essential health, even more important than healthy diet (which is already essential!). There is a relationship between light, magnetism, and water that I don’t understand scientifically, but I know the practical application is to get more sunshine/daylight (even when cloudy), grounding (bare feet on the ground), and drinking structured water.
Last year my husband installed a whole house water filtration system w/ a water structuring device (from Radiant Life) and my houseplants immediately improved and have done very well since.
Thanks again. I sure appreciate you taking time to find an answer for me!
Laurie A
Very interesting. More and more, I’ve been seeing how light affects us, both positively and negatively. We live in a rural area, with no outdoor lights, and no LED lights in our bedroom, but I’ve wondered how natural ambient light affects us, such as during a full moon. It seems our bodies would be made to handle this well, but I’m curious if this has been studied. We have wood blinds, but no curtains.
Jacqueline
Laurie A,
Yes, I am also wanting to know more about the moon’s effects at night. As an RN who worked ER and OR, we saw an effect on behavior.. the word lunatic comes from lunar (moon). People were a bit crazier when there was a full moon, for sure. If you find anything, let us all know!
Blessings,
Jacque