How to Conduct a Personal Experiment: Yellow Light Exposure

You guys ready for another personal experiment? I hope so. Even if don’t think you’re ready to take something on, I’m confident you’ll be able to handle this one, because it’s relatively simple, intuitive, and easy. It’s also something I’ve been discussing for a couple years now, so you’re most likely familiar and comfortable with the concept. But most importantly, today’s experiment is a gentle one that requires very little commitment. No jumping in freezing cold water, no drastic changes to your sleeping schedule. All I’m asking you to do is experiment with nighttime yellow light exposure.


Remember how I wrote about nighttime exposure to blue light affecting melatonin secretion and, subsequently, sleep quality and duration way back when? Yeah, that. In case you didn’t read it, I’ll give a quick explanation:

The color (or wavelength) of the light we perceive entrains, or “sets,” our biological clocks, also known as circadian rhythms. And in the natural environment, with its reliably consistent lighting schedule, it works pretty well. During the day, we see all the visible wavelengths provided by the sun, including violet, blue, green, yellow, orange, and red light, and this “tells” our bodies that it’s daytime, that’s it’s time to be active, to hunt, to gather, to build, to work, to exercise, to study, and so on. Secretion of melatonin, the “sleepy hormone,” is blunted. At night, when the only visible light is historically the longer wavelengths, the yellows, oranges, and reds which we create through campfires, or candles, or gas lamps, melatonin secretion is unaffected. We get sleepy like we should, when we should. All is well. And if we had evolved to be nocturnal, like rats, all that low-wavelength blue light exposure available during the day would let us know it’s time to sleep.

But we don’t use candles and oil lamps at night anymore, do we? We use white LED lights (blue light) and stare into laptop (blue light) and high-def TV (blue light) screens. We use our iPhones (blue light) or Androids (blue light) in bed, even waking up in the middle of the night just to check our email (blue light) because “why not, we’re up anyway!” To really get a sense of this, next time you take a nighttime stroll around your neighborhood, pay attention to the living rooms of the houses you walk past. If they’re got their plasma or LCD going, the lights off, and it’s dark out, the entire room will be bathed in an overpowering blue light. It’ll look like a scene from an alien abduction movie or something. Of course, whether the room lights are on or off, that blue light from the screen is still there, beaming directly into the eyes of those present and affecting the secretion of their melatonin.

And we wonder why we have so much trouble getting good sleep.

It’s not just sleep that’s affected (although that’s enough reason to take heed). Disturbing our circadian rhythms with improper light exposure may have a range of other health effects, including, but not limited to:

When I say “yellow light exposure,” what I’m really talking about is “blue light avoidance.” Today, I’m going to show you how to put together an experiment to test the effects of exposing yourself to yellow/orange/red light and avoiding blue light. Although that sounds like we’re testing two things, we’re really not, since yellow light has little to no effect on our melatonin production. For all intents and purposes, it and other, higher wavelength lights are neutral, while blue light is antagonistic to our circadian rhythms.

Okay, so how do I do it?

First, choose a goal that yellow light exposure and blue light avoidance might help make possible. Since we can’t really test our susceptibility to cancer in a short trial, nor does metabolic syndrome develop in mere weeks, let’s test the effect of yellow light exposure (and blue light avoidance) on some aspect of our sleep.

  • “I want to feel more refreshed in the morning.”
  • “I want to go to bed by 10 PM every night.”
  • “I want to have more melatonin in my morning urine” would be an effective way to test, but it also requires being able to test your urine for melatonin. Most people don’t have that on hand. Let’s go with the second one – getting sleepy and in bed by 10 every night – since that’s easy to quantify (did you go to bed at 10?). If you already go to bed at 10 PM, choose a time that’s earlier than your normal bedtime. The key is to find out if you get sleepy earlier without the blue light.

Come up with a hypothesis, such as:

  • “Since blue light exposure suppresses normal melatonin secretion, and melatonin makes us sleepy, exposing myself to yellow light and avoiding blue light after dark will help me get sleepy and go to bed by 10 PM.”

Next, identify some of the variables that could affect the results of your experiment:

  • Activity at night – Does what you do while avoiding blue light affect your sleepiness? Is reading more stimulating than hanging out with your spouse?
  • Electronics usage – Assuming you’ve taken steps to eliminate blue light exposure (wearing blue light-blocking goggles, installing f.lux on your computers), do electronics still stimulate you and keep you awake?
  • Yellow light source – Does a yellow light bulb act differently on you than candlelight?

Then, let’s take some measurements. What should you be measuring?

  • Bedtime – When did you get into bed?
  • Sleepytime – When did you start getting noticeably sleepy? Time of first yawn?
  • Sleeptime – When did you fall asleep? This is hard to measure without equipment, but you can probably approximate it. Personally, once I get too tired to read another page and find myself nodding off in the middle of a sentence, that’s my sleeptime, because I fall asleep as soon as I put the book down and turn off the (yellow) light.
  • Morning wakefulness – On a scale of 1-10, how awake and refreshed do you feel in the morning?

After a few weeks you should have enough data to start making some observations about what does and doesn’t work for you, and from there you can decide on what to test next, if anything. That’s it for this experiment. Have fun!

Thus concludes this short but sweet series on self-experimentation. What’d you think? I for one am a firm believer in the power of the self-experiment. In fact, I think it’s the ultimate arbiter of an individual’s ideal path to health. Sure, you could read all the blog posts and studies and papers and research in the world, but if you personally experienced results that completely contradicted the advice of the experts, what would you do? Would you continue down the path that supposedly worked for this cohort or that quadrant of some population somewhere? Or would you stick with what worked for you?

Exactly; we are complex beings with physiological processes that even the experts who study them for their entire lives don’t fully understand. Everyone is different, and there are no real one-size-fits-all plans – not any honest ones, at least – and so self-experimentation (even if it’s just an informal thing) is absolutely crucial and highly effective.


Now let’s hear from you. If you have any questions or comments about this self-experimentation series, leave them in the comment section and if there are enough questions, I’ll do a Dear Mark on self-experimentation next week. Thanks for reading!

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending more than three decades educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates flavorful and delicious kitchen staples crafted with premium ingredients like avocado oil. With over 70 condiments, sauces, oils, and dressings in their lineup, Primal Kitchen makes it easy to prep mouthwatering meals that fit into your lifestyle.

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