It’s hard to imagine a life without instant, unfettered, 24-hour access to light. In the 100-plus years since electric light became widely available, it has brought a cascade of fundamental change to the way we live. Electricity and artificial light have been, and continue to be, responsible for profound cultural, economic and technological progress.

But there is also a dark side to the abundance of artificial light in our lives. Exposure to light—particularly exposure at night, when our ancestors would have been dwelling in darkness—have resulted in profound changes to human behavior and created new risks to human health.

Scientists are still in the relatively early stages of identifying and understanding the ways that nighttime light exposure affects human health. Research shows that light exposure at night interferes with both the soundness of sleep and with sleep duration. Both sleep quality and sleep quantity are critical for maintaining health. Poor quality and insufficient sleep are strongly linked to increased risks for many forms of chronic and serious disease, including heart disease and stroke, diabetes, depression and other mood disorders, and some cancers.

Nighttime light exposure also suppresses the hormone melatonin, which plays an important role in regulating sleep and circadian rhythms. Light exposure during nighttime hours causes shifts to the timing of circadian rhythms, initiating a reset of the body’s biological clock that can contribute to a wide range of health problems.

Circadian rhythms, which function under the regulation of the body’s biological clock, play a significant part in controlling sleep and wake cycles. But the influence of circadian rhythms goes well beyond sleep. These biological rhythms regulate nearly every aspect of our physical and mental functioning, from metabolism and immune function to cognitive abilities to mood, and much more. When circadian rhythms are thrown out of sync—as they can be by nighttime light exposure—we face greater risks for a number of common health problems.

Blue wavelength light is especially disruptive to circadian rhythms and melatonin, and is now considered by scientists to be particularly hazardous to health. Blue light is a short wavelength light that is found in higher concentrations many modern light sources, including energy-efficient light bulbs and digital screens. These technological innovations, however welcome in other ways, have increased human exposure to what can be the most aggressively disruptive form of artificial light. A Harvard University study showed that blue wavelength light inhibited melatonin for approximately twice as long as green light, and also caused a twice-greater shift in the timing of circadian rhythms.

Let’s take a look at what we know about five common health conditions that science has linked to nighttime light exposure.

Obesity

Nighttime exposure to artificial light has been linked to increased incidence of obesity. A 2012 study of older adults in Japan found a significant association between higher body weight and the intensity of exposure to artificial light at night. Significantly, this association existed independent of melatonin levels, indicating that there may another mechanism by which light contributes to increased body weight. Higher levels of nighttime light exposure were also linked to larger waist circumference and a greater likelihood of obesity. Over the past four decades, obesity and exposure to artificial light at night have increased at strikingly similar rates, according to research.

Diabetes

Diabetes is a metabolic disorder that affects more than 29 million Americans. A compelling body of scientific research indicates that nighttime light exposure disrupts metabolic function, at least in part as a result of disruption to circadian rhythms. Circadian rhythms exert a powerful influence over metabolic processes. They help to regulate insulin levels and glucose tolerance, factors that contribute to diabetes and pre-diabetes. Research shows connections between diabetes and circadian rhythm disruption, as well as between diabetes and nighttime light exposure. Shift workers, who are often exposed to artificial light at night and contend with ongoing disruption to circadian rhythms and biological clocks, face higher risks for diabetes, according to numerous studies.

High blood pressure

More than 70 million American adults—roughly 29 percent of the population—suffer from high blood pressure. Treatment rates have climbed, but millions of Americans still aren’t receiving treatment for their high blood pressure. Recent research links nighttime light exposure to high blood pressure, likely through light’s suppressive effects on melatonin.

One study of 116 adults ages 18-30 exposed one group of the participants to regular room light and another group to dim light for an eight-hour period before bed. Scientists measured melatonin levels at frequent, regular intervals throughout this period. They found that the group exposed to room light experienced significant suppression of melatonin, compared to the dim light group. When this group was exposed to room light during actual sleeping hours, their melatonin levels were inhibited by more than 50 percent. Scientists who conducted the study posited that the suppression of melatonin may contribute to high blood pressure, as well as to diabetes.

Blood pressure fluctuates over the course of a 24-hour day and night, and like so many other of the body’s processes, is regulated by circadian rhythms. Blood pressure is typically higher in the daytime and lower throughout the night. Melatonin, released by the body during evening hours, has a lowering effect on blood pressure.

But there is likely more to connect nighttime light exposure to melatonin disruption. Other research indicates that light exposure at night increases blood pressure levels independent of melatonin levels.

Breast cancer

A growing body of research indicates that nighttime light exposure may be linked to several forms of cancer, including breast cancer. An international study that included 164 countries found that breast cancer risk was elevated by 30-50 percent in nations with the highest levels of nighttime light exposure, compared to nations with the lowest.

Research also indicates that nighttime exposure to bright light may accelerate breast cancer tumor growth, and that even dim light exposure at night may lead some types of breast cancer tumors to become resistant to some forms of treatment. Breast cancer risk, like diabetes risk, is also higher among women who perform shift work—a risk many scientists attribute to melatonin suppression and circadian rhythm disruption.

Cardiovascular disease

Light exposure at night is linked to several conditions that in turn contribute to cardiovascular problems. Evening light exposure disrupts and minimizes sleep. Both insufficient sleep and poor quality sleep are linked to increased risks for cardiovascular disease. Changes to melatonin levels—which occur as a result of nighttime light exposure—may contribute to heart disease, as well as to higher cholesterol levels and high blood pressure. Obesity, which is linked to evening light exposure and to circadian rhythm disruption, is a significant risk factor for heart disease. Evening light exposure also is linked to metabolic dysfunction. Metabolic disorders such as metabolic syndrome increase risks for cardiovascular disease.

Reducing the risks

It doesn’t take prolonged or intense exposure to nighttime light in order to cause problems. As some of the research I’ve discussed shows, even dim light can interfere with circadian function and sleep, triggering increased risks for disease. The light of a table lamp is bright enough to have a significant impact, say scientists.

There are steps you can take to minimize the negative effects of nighttime light exposure.

Avoid unnecessary and excessive exposure to evening light. This doesn’t mean you need to plunge yourself into complete darkness at sundown. But become aware of your nighttime environment, and look for ways to reduce the amount of artificial light within it. Close your curtains to block the streetlight that shines in your window. Use dimmer switches to lower levels of light throughout the house after dinner.

Block bright and blue light on screens. Increasingly, digital devices are equipped with blue-light blocking filters and timers to reduce brightness at night. There are also several apps available that perform these functions. In addition, you can attach blue-light blocking filters directly to screens themselves, or use blue-light blocking glasses.

Power down. I recommend a Power Down Hour before bedtime. Spend the 60 minutes before your head hits the pillow away from television, phone, and other screens and electronics.

Get Better Bulbs. A Power Down Hour doesn’t mean you have to sit in the dark. There are now commercially available light products that will filter out the “blue spectrum” of light which causes most of the problem, while still providing enough light for reading or other activities.

Sleep in the dark—and avoid middle-of-the-night light exposure. Take steps to ensure that you’re sleeping in the dark, including using timers on bedroom lights and devices, and covering windows to block outside light. Even a fleeting burst of exposure to light during the night can throw your circadian rhythms off track. Install dim, or speciality nightlights in bathrooms or hallways to avoid having to turn on other lights.

Get plenty of light exposure throughout the day. Light exposure during the day boosts attention and alertness, improves mood and cognitive function, strengthens circadian rhythms and can help you sleep better at night.

Light can provide terrific benefits to health, when used in the right ways. Light therapy is used to treat a range of conditions, from sleep problems and jet lag to depression and dementia. In order to reap the many benefits of artificial light and minimize the hazards it can pose, we need to become savvy, thoughtful consumers of light. Taking steps to reduce and eliminate nighttime exposure to artificial light is an important step in protecting health in our modern, lights-always-on age.

Sleep Well, Be Well,

Michael Breus, PhD