Light Exposure at Night Can Destroy Your Thyroid

Light Exposure at Night Can Destroy Your Thyroid by Dr. Joseph Mercola for Mercola

Exposure to light at night is a recent phenomenon that increased dramatically after the invention of electric lighting. Human bodies have not entirely adjusted to this change, and still run on a 24-hour cycle, or circadian rhythm, which includes regular cycles of light and dark.

When you’re exposed to light at night — a time when your body expects it to be dark — physiological changes occur. Inside the suprachiasmatic nucleus (SCN) of your brain, which is part of your hypothalamus, resides your master biological clock. Based on signals of light and darkness, your SCN tells your pineal gland when it’s time to secrete melatonin — promoting sleep — and when to turn it off.

Exposure to light leads to advances or delays in your circadian rhythm, known as phase shifts. Typically, exposure to light early in the morning causes a phase advance, which leads to earlier waking. Light exposure at bedtime will lead to a phase delay, or later wakening.

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Nighttime exposure to light inhibits the secretion of melatonin, which can cause circadian disruptions that play a role in cancer.1 In fact, it’s previously been shown that higher exposure to outdoor light at night may increase the risk of postmenopausal breast cancer,2 and emerging evidence suggests light at night may increase thyroid cancer, too.3

Light at Night Increases Thyroid Cancer Risk

It’s believed that both breast cancer and thyroid cancer “share a common hormone‐dependent etiology,” while thyroid function is also regulated by circadian rhythm. These two factors led researchers from The University of Texas Health Science Center at Houston School of Public Health to evaluate whether exposure to light at night plays a role in the incidence of thyroid cancer.

The study followed 464,371 participants in the National Institutes of Health-American Association of Retired Persons Diet and Health Study for an average of 12.8 years.4 Satellite data was used to estimate nighttime light exposure, which was then linked to residential addresses, while thyroid cancer cases were followed via state cancer registries.

Adjustments were also made for other contributors to thyroid cancer risk, including sociodemographic, lifestyle and environmental factors. A positive association was found between light exposure at night and thyroid cancer risk, with those in the highest quintile of nighttime light exposure having a 55% increased risk of thyroid cancer compared to those in the lowest quintile.

“The association was primarily driven by papillary thyroid cancer and was stronger in women,” the researchers noted. “In women, the association was stronger for localized cancer, whereas in men, the association was stronger for a more advanced stage. Results were consistent across different tumor sizes.”5

The study is observational and therefore doesn’t prove causality, however the findings suggest additional research is warranted. Study author Qian Xiao, Ph.D. said in a news release:6

“[W]e don’t know if higher levels of outdoor light at night lead to an elevated risk for thyroid cancer; however, given the well-established evidence supporting a role of light exposure at night and circadian disruption, we hope our study will motivate researchers to further examine the relationship between light at night and cancer, and other diseases.”

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