06/27/2025
Our Paleolithic Relationship with Light
Ffej Onsark awoke each morning as the first rays of sunlight stretched across the savannah. His internal clock, governed by natural light cycles, was in perfect sync with the rising and setting sun. Exposure to morning light, rich in blue wavelengths, triggered an endocrine response that roused Ffejâs body from his grass mat and signaled that it was time to be alert, active, and engaged with the world around him. At night, as darkness fell, the warm, amber glow of a fire provided just enough illumination for his tribe to gather without disrupting the natural progression toward rest.
In Ffejâs world, the day rounded to a close as the sun dipped below the horizon. Unlike the perpetual glow of modern life, Ffejâs nights were illuminated only by the flicker of firelight or the soft glow of the moon. This dim, warm light allowed his body to gradually wind down, signaling to his brain that it was time to repair, regenerate, and dream. For hundreds of thousands of years, this seamless tango with natural light ensured that Ffejâs circadian rhythm was in perfect harmony with the environment.
Circa means âapproximately.â Dia translates as âday.â Ffejâs relationship with light governed his sleep-wake cycle across approximately a day.
But what was once an evolutionary adaptation to the cycle of light and darkness has been hijacked by the relentless march of technology. Today, we live in a world where the days end but the night never truly falls.
The Glow of Modernity: The Evolution of Artificial Light and Blue Light Exposure
The invention of artificial light begins with the candle in 3000 BCE. The Ancient Egyptians fashioned simple torches made by soaking reeds in melted animal fat, without a wick. They were more like oil lamps than modern candles, but they served a similar functionâproviding light even after the sun set.
But the story of how humanity fell out of sync with the natural cycles of light really begins with the invention of the light bulb in 1879. Thomas Edisonâs incandescent bulb was a revolutionary innovation, transforming how people lived and worked. For the first time, quotidian activities were no longer restricted to daylight hours, and we could extend both our productive and social lives into the night. All night if we were so inclined.
Edison was no admirer of sleep, once uttering, âSleep is a criminal waste of time and a heritage from our cave days." He espoused the 20th-century belief that sleep was an obstacle to productivity and considered it an outdated necessity. Unfortunately, this mindsetâand his inventionâhas played a significant role in disrupting our natural sleep patterns that evolved over thousands of years.
That said, the light emitted by incandescent bulbs was relatively warm, closer to the infrared end of the visible spectrum, more like the fire around which Ffej and his gathered. Thus, the glow of the early lightbulb had a limited impact on our circadian rhythm because it didnât mimic the blue wavelengths of natural sunlight.
It wasnât until the invention of fluorescent lighting in the early 20th century that blue light exposure began to increase significantly. Fluorescent lights emit a broader spectrum of light, including âbluerâ wavelengths closer to ultraviolet radiation, and became a standard in workplaces, schools, and public spaces. While they were more energy-efficient than incandescent bulbs, their cooler light started to affect our exposure to blue wavelengths well beyond the natural daylight hours.
Subscribe
The next seismic shift came with the rise of light-emitting diodes (LEDs) in the 1990s. LED lights are incredibly efficient, long-lasting, and versatile. They are an environmental boon, but they also emit a high concentration of blue light. As LEDs became the standard in homes, offices, streetlights, and electronic devices, blue light exposure increased dramatically. Unlike incandescent bulbs that had a warmer, softer glow, LED lighting brought a brightness that mimicked daylight, even during the night.
The true tipping point, however, came with the proliferation of digital screens. The launch of personal computers, the spread of the internet, and the rise of smartphones, tablets, and televisions introduced a constant source of blue light into our daily lives. Each of these devices emits a significant amount of blue light, designed to make screens appear brighter and more readable in various lighting conditions.
The rise of 24/7 accessibility to information, entertainment, and communication has fundamentally changed our relationship with light, turning our evenings into perpetual daylight.
Letâs consider the timeline of these technological advancements:
In 1995, the internet became widely accessible to the public, and personal computer use soared, bringing blue light into homes and workplaces on a massive scale.
In 2007, Apple released the first iPhone, sparking the smartphone revolution. Suddenly, we had bright, blue-light-emitting screens in our hands at all hours of the day and night. By 2021, more than 3.8 billion people worldwide owned a smartphone.
In the 2010s, the explosion of streaming services like Netflix, Hulu, and Amazon Prime made endless entertainment available on demand. Screen time soared, with the average adult spending over 11 hours per day interacting with screens by 2020. This shift marked an unprecedented rise in blue light exposure, well into the night. Personally, I blame Larry David for my intermittent insomnia. I love his comedic antics, but the more I learned about binging TV at night, the more it curbed my enthusiasm for bedtime streaming.
LED televisions, laptops, tablets, e-readers, and even smart home devices like Alexa or Google Home feature blue-light-emitting screens that we interact with constantly. The bright white LEDs used in these devices are specifically designed to enhance screen visibility, clarity, and sharpness, but at the cost of our circadian health.
The outcome of this technological evolution is that the average person is now exposed to ten times more blue light than just a few decades ago. What was once a natural cue to wake up and start the day has now become an all-day, all-night assault on our physiology.
The Hard Science: How Blue Light Disrupts Our Circadian Rhythm
How does this all work?
Exposure to blue light, specifically in the range between 460 nm and 484 nm, sets your circadian clock. When humans get morning light, blue light radiation enters the eye and interacts with specialized neurons in the inferior part of the retina known as intrinsically photosensitive retinal ganglion cells. These sensory neurons evolved in the lower part of the retina because light from the sun is coming from above, the superior field.
These cells then send a message to your bodyâs master clock â a pair of small nuclei in the hypothalamus of the brain, above the optic chiasma, known as the suprachiasmatic nucleus. The suprachiasmatic nucleus regulates the flow of hormones in your body across approximately a day. Specifically, it signals the pineal gland to produce and secrete the hormone melatonin at a certain time. Melatonin naturally induces grogginess.
When your eyes get blue light in the morning, your circadian clock is set such that about 12-14 hours later, melatonin will begin to pulse through your bloodstream and round your day with sleep.
The evening fire emitted a warm glow from the ground â the inferior field. These light waves had no hormonal impact on the body. This is why amber nightlights at ground level donât disrupt sleep.
The impact of artificial blue light is a direct attack on our biology. Quite literally, itâs an endocrine disruptor as it upsets the natural crests and troughs of cortisol and melatonin. What happens when these sensory neurons in our eyes are exposed to blue light?
A number of things. None of them good if triggered at the wrong times.
Exposure to blue light suppresses melatonin production, leading to delayed sleep onset, reduced sleep quality, and less time spent in REM sleep. And, at the same time, blue light exposure elevates cortisol, the stress hormone, which is supposed to taper off in the evening. Elevated cortisol levels at night result in restlessness, anxiety, and increased wakefulness, contributing to a vicious cycle of stress and poor sleep.
Poor sleep doesnât just make you feel tired and cranky. Good sleep is foundational to health.
The circadian rhythm plays a crucial role in regulating metabolism, including how our bodies process glucose. Exposure to blue light at night disrupts insulin sensitivity, leading to higher blood glucose levels and an increased risk of developing type 2 diabetes. Research has shown that people who are exposed to blue light at night tend to have higher body mass indices (BMIs) and are more prone to weight gain.
The relationship between increased exposure to light, less sleep, and fat stowage is actually adaptive. The longer summer days delay the production of melatonin resulting in less sleep, which, in turn, produces some degree of insulin resistance. When your body is insulin resistant, it stores fat. In Paleolithic times, it was advantageous to store fat in the late summer as the paucity of winter was around the corner. When the body experiences a dearth of calories, it will burn fat for energy.
But, as addressed in the chapter on food, winterâs fallow never arrives. So, we have this insidious collusion between poor sleep, insulin resistance, fat stowage, and endless calories.
Disrupted sleep due to blue light exposure has been linked to elevated blood pressure, increased heart rate, and a higher risk of cardiovascular disease. The circadian rhythm governs the cardiovascular system's functioning, and chronic misalignment contributes to inflammation and arterial stiffness.
Melatonin is most celebrated as a sleep hormone, but it wears other hats. Itâs a potent antioxidant and immune modulator. Its suppression means that the bodyâs ability to repair and defend itself is compromised, leading to a weakened immune system and increased susceptibility to infections and diseases.
An Evolutionary Mismatch: Innovation versus Adaptation
Our circadian rhythm evolved under conditions where light was a reliable indicator of day and night. Today, artificial lighting and screens have severed this connection, making our bodies believe that night never falls. This mismatch between our evolutionary design and our current environment has led to an epidemic of insomnia, metabolic disorders, and chronic disease.
In fact, studies have shown that night shift workers, who are exposed to artificial light at night, have a 40% increased risk of cardiovascular disease and are more likely to suffer from obesity, type 2 diabetes, and cancer. These health outcomes are not unique to night shift workers; anyone who engages with screens late into the evening experiences a similar disruption.
Subscribe
Realigning with Our Evolutionary Engineering: Light Hygiene Practices
To combat this mismatch, we must actively seek to realign our lifestyles with the light-dark cycles that shaped our evolution. In my book, Good Stress, I go deep into the human relationship with light. I unpack not only the protocols that realign us with our circadian rhythm, but also how the human relationship with light impacts our ability to produce endogenous Vitamin D and intercellular antioxidants.
Here, I will share some basic practical steps to reconnect with our biological wiring:
#1: Get Morning Light Exposure:
Within 30 minutes of waking, spend at least 20 minutes outside to expose your eyes to natural morning light. This exposure sets your circadian clock, ensuring melatonin production begins at the appropriate time later in the evening. A light intensity of around 10,000 lux is ideal to trigger your SCN effectively.
#2 Blue Light Blocking at Night:
Limit screen time at least 1 hour before bed. Use blue light filters on your devices or wear blue light-blocking glasses that filter wavelengths between 400â500 nanometers.
Use amber-hued bulbs or incandescent lighting in the evening, which mimics the color of firelight and does not interfere with melatonin production.
#3: Maximize Darkness During Sleep:
Ensure your sleeping environment is completely dark. Use blackout curtains, turn off electronic devices, and consider using an eye mask to minimize any exposure to ambient light.
#4 Respect Your Melatoninâs Counterpart â Cortisol
Engage in stress-reducing activities in the evening, such as meditation, gentle stretching, or deep breathing exercises, to help cortisol levels taper off and allow melatonin to rise naturally.
The Bottom Line: Living More Like Ffej
Our circadian rhythm is more than just a sleep-wake cycle; itâs a master regulator of countless biological processes. It influences hormone production, metabolism, immune function, and even gene expression. When we bombard our brains with blue light at night, we create a state of perpetual âdaytimeâ that disrupts this intricate hormonal symphony.
Ffejâs life was marked by periods of intense activity and deep rest, guided by the light of the sun and the darkness of night. He didnât require coffee to wake up and then another caffeine boost at 4 pm to make it through to Netflix Oâclock. Our modern world, with its 24/7 glow, has severed us from this natural rhythm. Weâve traded deep, restorative sleep for endless scrolling and binge-watching.
To reclaim our health, we must realign our exposure to light with the evolutionary blueprint that served us. Once again, we must live a little more like Ffej.
In essence, the solution is simple: when night comes, let it be night. By embracing this primal relationship with light, we can reconnect with our evolutionary roots, allowing our bodies to function as they were designedâhealthy, resilient, and in harmony with the environment.
We may never live on the Serengeti, but we can still bring a little bit of Ffejâs world into our own. And in doing so, we may find that the key to modern health lies not in the next technological breakthrough, but in honoring the rhythms that have been with us all along.
Words by~Jeff Krasno
