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.
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.”
Strong Link Between Thyroid Cancer and Circadian Disruption
Thyroid cancer is the most common cancer of the endocrine system, and rates have increased significantly in the last few decades.7 Insulin resistance is one of the most significant, and modifiable, risk factors,8 but increasing attention is also being given to environmental factors like circadian clock disruption.
Your body’s 24-hour circadian clock regulates many physiological functions — endocrine rhythms among them. Writing in the journal Cancers, Italian researchers suggested that one way circadian disruption may be linked to thyroid cancer is by inducing insulin resistance:9
“Disruption of the circadian timing system caused by circadian misalignment such as shift work, chronic jet lag, high fat intake, inappropriate eating times, and abnormal sleep patterns could be responsible of insulin resistance, diabetes mellitus type 2, obesity, metabolic syndrome, cardiovascular diseases and several types of cancers, including TC [thyroid cancer].
Conversely, proper coordination of circadian behavior and sleep homeostasis may improve several conditions including insulin resistance and overall metabolic fitness.
The molecular mechanisms linking circadian clock disruption and TC are still unknown but could be, at least in part, insulin resistance. Indeed, this metabolic alteration is associated with a well-known risk factor for TC i.e., hyperthyrotropinemia, which, in turn, has also been associated to sleep disturbances.”
They also pointed out a number of mechanisms that occur via circadian disruption that could contribute to thyroid disorders. Among them:10
Alterations in the rhythmicity of thyroid stimulating hormone (TSH) secretion
Disruptions to hypothalamic-pituitary-thyroid (HPT) axis function
Modifications in genes controlling the cell cycle, apoptosis, DNA damage, inflammation and immune response
Strong changes in the expression of variants of various “clock” genes, including PER2–3, CRYs, BMAL1, REV-ERBs and RORs, have also been found on the transformation of thyroid nodules into cancer, and have even been suggested as biomarkers for use with thyroid nodules that could potentially be predictive of thyroid cancer.11
Clock genes are known to control rhythms that affect physiology and behavior,12 and may also be involved in cancer. Indeed, researchers wrote in Genome Medicine while exploring the many ties between circadian rhythms and disease, “Epidemiological studies have linked circadian disruption to increased cancer susceptibility in all key organ systems.”13
Light Exposure, Sleep Deprivation and Cancer
It’s more difficult to sleep well if you’re exposed to light at night, and the resulting sleep deprivation may also increase cancer risk. An association has been found between insomnia and thyroid cancer, for instance,14 with women with insomnia having a 44% increased risk of thyroid cancer compared to those without. Sleep deprivation is also linked to higher thyroid-stimulating hormone concentration, which in turn is linked to thyroid cancer.15
Further, in a study involving 1,654 adults from the Penn State Adult Cohort, those who slept less than six hours and had cardiometabolic risk factors (high blood pressure, elevated glucose or Type 2 diabetes) had an increased risk of dying from cancer, by 2.92 times.16
In relation to thyroid cancer, specifically, chronic sleep deprivation is linked to disruption of rhythmic thyroid stimulating hormone secretions, which is associated with an increased risk of thyroid cancer. Researchers continued in Cancers:17
“Furthermore, disruption of circadian rhythm has been linked to alterations in gene-related apoptosis, DNA damage, cell cycle, and stemness, and thereby to carcinogenesis … In light of this evidence, it is biologically plausible that circadian clock alterations could represent a potential risk factor of developing TC. However, so far, no epidemiologic study has been directly addressed in this relationship.”
Disrupted sleep also wreaks havoc on your metabolic health, which could indirectly increase thyroid cancer risk. Irregular sleep patterns increase the risk of metabolic syndrome by 23% for each one hour of sleep difference, such as going to bed earlier or later than usual; chronic one-hour loss increases the risk by 27%.18
Metabolic syndrome is characterized by three or more of these factors: a large waist circumference, high blood pressure, insulin resistance and/or high blood sugar, low high-density lipoproteins and high triglycerides. Since insulin resistance and related metabolic disorders are linked to thyroid cancer, this is another way that light exposure at night plays a role. Researchers noted:19
“Based on these observations, it is reasonable to expect that improving insulin resistance through synchronization of circadian rhythm or chronotherapy in conjunction with a healthy diet, physical activity and conventional anti-cancer therapies, could exert beneficial effects on prevention and treatment of TCs developed in insulin resistant patients with disrupted circadian rhythms.”
It’s likely that sleep disturbances induced by exposure to light at night may contribute to cancer via multiple mechanisms, including a suppression of immune function by disrupting circadian rhythms, reduced production of melatonin and promotion of inflammation.20
Light Pollution Could Lead to Dire Health Outcomes
Chris Kresser, an acupuncturist, licensed integrative medicine clinician and co-director of the California Center for Functional Medicine, is among those who have sounded an alarm over the health risks of exposure to light at night and light pollution.
Most of your endocrine hormones, including not only thyroid hormones but also growth hormone, cortisol, leptin, melatonin and insulin, have a daily rhythm that, when disrupted, may interfere with how your body functions.
According to Kresser, the most potent regulator of your circadian rhythm is exposure to light at the proper times and intensities — and vice versa, in that light pollution is a potent disruptor of your circadian rhythm that interferes with sleep, hormones, mood, cognition and more.
“Thyroid hormones have circadian rhythms, too,” he writes. “Sleep deprivation from ill-timed light is associated with abnormal thyroid function.” In addition, he cited 11 other health consequences of light pollution, which include:
Disruption of the hypothalamus-pituitary-adrenal (HPA) axis, which controls the stress response
Fertility and menstrual problems
Depression and mood disorders
Cognition and memory deficits
How to Reduce Light Exposure at Night
In the modern world, avoiding light exposure at night isn’t always as simple as turning off the lights. If your bedroom is affected by light pollution, be sure to use blackout shades to keep light out or wear an eye mask when you go to sleep. Remove all sources of light from your bedroom, including a digital alarm clock.
You should also swap out LED lights with incandescent bulbs, which are less efficient at suppressing melatonin, particularly in areas where you spend most of your time during the day and evening, such as your kitchen, bathroom and bedroom. Leave the LEDs for areas such as hallways, closets, garage and porch, where your exposure to them is minimal.
When it gets to be late afternoon and evening, wear amber-colored glasses that block blue light, and turn off electronics — or at least be sure to wear the glasses while you’re using them. You can also install blue light-blocking software like Iris on your computer, cellphone and tablet.21
Part of optimizing your circadian rhythm is avoiding light at night, but the flipside is also important: if you’re in darkness all day long, your body can’t appreciate the difference and will not optimize melatonin production. So, ideally, to help your circadian system reset itself for thyroid health and overall health and wellness, avoid light at night and get at least 10 to 15 minutes of light first thing in the morning as well.