Getting Too Much Bright Light at Night May Increase Your Cancer Risks. Here’s Why

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illustration of person scrolling through Instagram, unable to sleep, as the sun rises
Illustration by Mira Norian for Verywell Health

Key Takeaways

  • Research shows that exposure to bright lights at night—especially blue light—increases the risk of breast cancer.
  • Bright light suppresses melatonin production, a hormone that regulates sleep and possesses anti-cancer properties.
  • To protect yourself from these effects, you can use lights with zero blue content in the evenings, sleep in a dark room, and go for morning walks for daylight exposure.

Getting enough sunlight during the day is critical for good health. But when it’s time to wind down, bright light exposure can suppress melatonin production and even increase the risk of cancer.

Research has repeatedly shown that bright light exposure at night—especially blue light—increases the risk of prostate,1 colorectal,2 and particularly breast cancer.3

“Bright light exposure at night can suppress the body’s production of melatonin, a hormone that regulates sleep and possesses anti-cancer properties, thereby potentially increasing the risk of breast cancer,” Yong Zhu, PhD, the assistant director of the Yale Cancer Center for Global Cancer Epidemiology, told Verywell.

The International Agency for Research on Cancer (IARC) classifies night shift work, which involves exposure to high levels of light at night, as a probable carcinogen.

A 2017 study found that long-term rotating night shift work was associated with a higher risk of breast cancer, particularly among women who performed shift work during young adulthood.4 Another analysis from 2018 concluded that night shift work increases the risk of breast cancer in pre-menopausal women, particularly those with high intensity and long duration of exposure.5

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Why Might Bright Light Exposure Increase Cancer Risk?

Melatonin is the darkness hormone that makes sure your body’s circadian rhythms are synchronized to day and night time, and it acts as a suppressor of breast cancer tumors, according to Martin Moore-Ede, MD, PhD, a former professor at Harvard Medical School who wrote the book “The Light Doctor.”

“Bright light in the evenings, especially if it is rich in sky-blue wavelengths, is highly disruptive to the circadian system, causing circadian disruption and melatonin suppression,” Moore-Ede told Verywell. “These increase the risk of breast cancer and accelerate tumor growth.”

He added that people work night shifts, and those who stay up late on their phones—which emit blue light and are held close to the eyes—are more susceptible to cancer, especially hormone-sensitive cancers such as breast cancer and prostate cancer.

The problem is not bright light per se. What really affects your circadian clock lies in the blue light wavelengths between 440–495 nanometers (nm), which resemble natural daylight, Moore-Ede explained.

“We are more than 25 times more sensitive to the 440–495 nm sky-blue light than to full-spectrum white light,” he said. “The problem is that compared to the incandescents that are now banned, LEDs are much richer in the key blue wavelengths.”

Blue light at night can suppress melatonin secretion and shift circadian rhythms much more so than white light. Aside from hormone disruption, this type of light exposure can also disturb your sleep duration and quality, increasing the risk of health problems like depression and diabetes.6

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How Can You Reduce Bright Light Exposure at Night?

While many of the lights today tend to be rich in blue wavelengths, there are still ways to protect yourself from the potential effects.

Moore-Ede said you can opt for lights with zero blue content in the evenings, sleep in a pitch-black room, and go outside for an hour every morning for daylight exposure.

Getting blackout curtains or blinds might be worth it, Zhu said, especially if you’re sensitive to light or if you want to optimize your sleep quality by creating a darker sleep environment.

It’s also important to note that while evidence suggests a link between bright light exposure and a higher risk of breast cancer, more research is needed to understand the extent of this connection, she added.

“Factors like individual genetic susceptibility, duration of exposure, and other lifestyle elements also contribute to the complex interplay between light exposure and cancer development,” Zhu said.

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