BY MARTIN MOORE-EDE, MD, PHD

Wake up, Moore-Ede!” the Surgeon-in-Chief yelled. It was 3 am, and my head had bobbed as my hand slipped. I was a motivated and ambitious young surgeon, and being yelled at in the operating room by the Surgeon-in-Chief was not good for my career…or for the patient.

It was because my circadian sleepwake cycle had been scrambled after working a year of 36-hour shifts under bright hospital lights.

We should wonder: Why do 25 percent of us suffer from insomnia, and 35 percent of us are chronically sleep-deprived? How does using the wrong electric lights, like blue light, in the evening disrupt our sleep?

Much of it is because we have failed to use the right light at the right time of day.

This began a lifetime journey—and a personal mission—to understand circadian clocks, how they interact with light and shape our health. I left my career as a surgeon to enter Harvard Medical School’s PhD program to study the physiology of circadian rhythms. Then, as a professor at Harvard Medical School, I built a Laboratory for Circadian Physiology.

With my colleagues, we located the circadian clock—the body’s internal 24-hour timing system—in the human brain, discovered how it is synchronized by light, and revealed the role electric light at night plays in disrupting circadian rhythms and negatively affecting our health.

The key lies in how our bodies detect a specific signal hidden within ordinary light.

How Light Sets the Body’s Clock

The light that illuminates us, whether it is sunlight or electric light, is composed of a rainbow mix of colors that fuse to form the white or yellowish color we normally observe. We only get to see the beautiful array of the separated colors of the rainbow when sunlight falls on a piece of crystal or raindrops in the sky. Then we get to admire the violet, blue, green, yellow, orange, and red hidden in the visible light spectrum.

In the 1980s at Harvard, when we located the master circadian clock in the human brain and showed that it was synchronized by daily light-dark cycles, we assumed that any visible light would be effective.

At that time, it never occurred to us that only a narrow part of the light spectrum, a sky-blue color, would be responsible for synchronizing circadian clocks to the Earth’s 24-hour rotation. Nor did we realize that this sky-blue color would enable us to identify which electric lights are unhealthy.

In contrast, the ipRGC sky-blue receptors do not provide any visual perception, but quietly inform our circadian clocks of the time of day, without our ever being consciously aware of their powerful regulation of our brain and body.

When Blue Light Disrupts the Body’s Clock

When artificial light sources emit sky-blue wavelengths at night, we are messing with Mother Nature—often without realizing it. We cannot see with our eyes how much sky-blue light a white light bulb emits. We just see it as white light without being aware of the spectral rainbow of colors that make up that light.

While ipRGC receptors peak in sensitivity at about 480 nm, they respond strongly to a broader band of short-wavelength blue light (approximately 460–495 nm). Blue light most strongly synchronizes the body’s clock to the environmental day–night cycle, a process called entrainment.

When you are exposed to light containing potent blue wavelengths at 460–495 nm, it disrupts your circadian rhythms and impairs your sleep at night. The solution is to use blue-depleted light at night—i.e., light without those disruptive blue wavelengths—rather than conventional LED lights, which are rich in blue. This greatly reduces the risk of shifting circadian clocks and disrupting sleep.

Because we are so sensitive to blue light at night, a range of blue wavelengths can suppress pineal melatonin production. These blue wavelengths interfere with melatonin’s signaling of natural darkness and undermine melatonin’s important protective effects at night.

These wavelengths are far more potent than full-spectrum white light. Exposure to even a small amount of blue light at night is highly disruptive to circadian rhythms.

Using Light the Right Way at Night

What does this mean for how we light our homes at night? Research into circadian light interactions has made one thing clear: the spectrum of light we use in the evening matters.

During the three hours before bedtime, indoor lighting should contain as little blue light as possible. Blue wavelengths—particularly those in the 460–495 nm range—send a powerful daytime signal to the brain’s circadian clock. When these wavelengths reach the eyes at night, they suppress melatonin production and delay the body’s natural transition to sleep.

There are two ways to reduce this disruption. One approach is to dim indoor lighting in the evening so that fewer blue photons reach the eyes. While effective, dimming alone can make it difficult to read or complete normal evening activities.

The better solution is to reduce the blue content of evening lighting while maintaining sufficient brightness for visual tasks. By replacing conventional blue-rich LED lights with blue-depleted lighting after sunset, it is possible to maintain comfortable illumination while protecting the body’s circadian timing.

What ultimately matters is not how warm or cool a light appears, but how much blue light it delivers to the eyes. The lighting industry often promotes correlated color temperature (CCT) as a measure of circadian friendliness, but this can be misleading. Lights that appear very different in color can still emit similar levels of blue light at the key circadian wavelengths.

For circadian health, the critical factor is controlling the amount of blue light reaching the eyes in the evening.

The principle itself is simple. The circadian system evolved under a predictable pattern of light: bright blue-rich daylight and darkness after sunset. Modern lighting has blurred that boundary. By restoring the natural pattern—bluerich light during the day and blue-depleted light at night—we allow the body’s internal clock to function as it was designed.

It is not blue light itself that harms us—it is exposure to blue light at the wrong time of day.

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Martin Moore-Ede, MD, PhD, is a leading world expert on circadian clocks, light, and health. As a professor at Harvard Medical School, he led the team that located the human circadian clock and showed it was synchronized by light. He founded the Circadian® Light Research Center, which identified the key blue wavelengths that control the circadian clock, and patented and developed the first evidence-based healthy circadian lights. He has appeared on the Today Show, Oprah, CNN, BBC, and NPR.

Well Being Journal adapted the above excerpt from The Light Doctor: Using Light to Boost Health, Improve Sleep, and Live Longer by Martin Moore-Ede, MD, PhD. Copyright © 2024 by Circadian Light Research Center. Reprinted with permission from the author.