June 19, 2017 -- Peek into the typical American household after dinner and you’ll find the occupants bathed in a faint bluish glow. As parents fire off late emails on their laptops or lie in bed with eyes fixed on e-readers, kids update their Snapchat accounts or squeeze in one last game on their phones. Even if the gadgets are off, new eco-friendly street lamps, TVs, and household bulbs shine into the night, emitting a brighter, shorter-wavelength (more bluish), and more potent light than older incandescent bulbs did.
All that concerns Charles Czeisler, PhD, MD, chief of the Division of Sleep and Circadian Disorders at Brigham and Women’s Hospital in Boston.
“The more research we do, the more evidence we have that excess artificial light at night can have a profound, deleterious effect on many aspects of human health,” says Czeisler, who is also director of sleep medicine at Harvard Medical School. “It is a growing public health concern.”
Czeisler is among a growing number of physicians, researchers, and health policy makers sounding the alarm that dark nights -- like a healthy diet, regular exercise, and good sleep habits -- are a key, endangered ingredient for long-term health.
Last year, the U.S. National Toxicology Program convened a 2-day workshop to explore mounting research linking exposure to artificial light at night not just to sleep problems, but also to weight gain, depression, cancer, and heart disease. In October, NASA went so far as to change all the lights on the International Space Station to ones that, as night falls, dim and change to longer-wavelength light, which has been shown to have less impact on human physiology than “blue light.”
Last June, the American Medical Association chimed in. It issued a statement showing concerns that the new ultra-bright light-emitting diode (LED) lamps many cities are now using in their streetlights could “contribute to the risk of chronic disease.”
Much of the research so far has been done on animals or comes from large population studies, which show patterns but don’t confirm cause and effect. But many health experts say the results are troublesome enough to warrant action now.
“As opposed to the many other kinds of harmful environmental pollutants out there, we are rapidly figuring out exactly what to do about this one, and it is really not that hard,” says Richard Stevens, PhD, a University of Connecticut cancer epidemiologist and light-at-night researcher. Just dim the lights at night and tone down that blue, he says.
The Power of Light
Light is by far the most important synchronizer of human circadian rhythms, or body clocks, Czeisler says. Specialized cells in the retina are finely tuned to respond to the short-wavelength light that comes from a cloudless blue sky. As light rays hit those cells, they tell the brain to stop pumping out drowsiness-inducing melatonin and start making hormones like cortisol and ghrelin that wake us up and make us hungry.
At dusk, in an electricity-free world, the opposite happens. As light fades, the body begins to transition to “nighttime physiology,” in which melatonin levels rise, body temperature drops, sleepiness grows, and hunger goes away. The time spent in this restful state, even if we are not actually sleeping, is restorative, Stevens says. Trouble is, in the modern world, we are bathed by lights that have the same potent wavelength that wakes up to, so our transition to nighttime physiology has been delayed by hours.
As Stevens puts it, we are “darkness deprived.”
The best-documented consequence, by far, of excess evening light exposure is short-term sleep disruption. In one study, people in a sleep lab who read from an e-reader at night saw their nighttime melatonin levels drop by 55% after 5 days, took longer to fall asleep, had less restorative rapid eye movement (REM) sleep, and felt more groggy the next day than those reading a paper book.
Another study, looking at teenagers, suggests they may be even more sensitive to light at night. Just an hour of exposure from a glowing device, like a phone, suppressed melatonin by 23%; 2 hours decreased it by 38%.
Sleep issues aside, light at night is now being accused of helping fuel weight gain and metabolic diseases. Studies show that people exposed to more bright light at night are hungrier and produce less insulin, making it harder for them to turn those late-night snacks into fuel. As a result, it rests in the blood, where it makes diabetes more likely, or it's stored as fat.
One March 2016 study by University of Haifa researchers compared World Health Organization obesity data with military satellite images of nighttime lighting and found that the men and women who lived in the places most illuminated at night were also the most likely to be obese.
Animal studies at Ohio State University show that even exposure to relatively dim light -- about the brightness of a child’s nightlight 3 feet from the eyes -- over 8 weeks has a measurable impact on the brain. It raises inflammation and lowers levels of a hormone that's key for promoting new brain cell growth. It also causes transmitters between neurons to whither.
The animals also showed “depressive like symptoms” and had memory problems, says study author Randy Nelson, chairman of the department of neuroscience at Ohio State University. While studies looking at the way light at night affects the human brain are only in their infancy, population studies of emergency room workers and oilfield workers chronically exposed to bright light at night show similar thinking and mood impairments, even if those workers are getting enough sleep, Nelson says. “This is not just a sleep problem. This is a problem of disruption of the entire circadian clock, and sleep is just one hand of that clock.”
Research is young, but some studies suggest that chronic exposure to excess light at night may also fuel cancer, in part by lowering the levels of melatonin -- a known anti-cancer agent -- circulating in the blood. Female night shift workers have a 50% to 70% greater chance of developing breast cancer during their lifetime, says David Blask, MD, associate director of the Tulane University Center for Circadian Biology.
One recent study of 75,000 nurses, published in the American Journal of Preventive Medicine, found that those who worked the night shift for more than 5 years were 11% more likely to die early. Some European governments, with health risks in mind, now pay women night shift workers hazard pay.
“In a sense, with all our gadgets, we are all night shift workers to a degree now,” says Blask.
Less Light at Night, More in the Day
Mariana Figueiro, PhD, light and health program director at the Lighting Research Center in Troy, NY, stresses that in addition to minimizing bright blue-hued light -- especially from gadgets held close to the eyes -- at night, we should try to maximize the amount of bright light we get during the day.
“It not only makes you more awake and alert by day; research suggests it may also make you less sensitive to the negative health consequences of light at night,” she says.
Stevens says that beyond the 7 to 8 hours of sleep you try to get each night, you should make an effort to get 3 more hours of relative darkness. You don't need to live by candlelight after dinner, but it’s a good idea to dim the lights and steer clear of bright blue screens. Replace the lights in your bedroom and bathroom with dimmer, longer-wavelength lights, consider using blackout shades if streetlights shine into your window, and invest in an eye mask for when it’s time to go to sleep.
At a minimum, you’ll wake up feeling more refreshed. Best-case scenario: You’ll live longer.
Cultivate a Healthy Body Clock
Get plenty of natural light by day: Take a walk outside in the morning. At the office, put your desk near a window. Invest in a bright light -- one that delivers 1,000 lux (a measure of light intensity) or more of blue-hued light at eye level -- to put on your desk at work. Studies show that most office environments are too dim to stimulate the positive, alerting effects of light by day.
One 2014 study by Northwestern University researchers found that people who got most of their bright light exposure before noon were about 1.4 pounds leaner on average than those exposed to most bright light in the evening.
Start powering down at dusk: Use programs like f.lux and other apps that automatically shift screens on phones and laptops to more orange-red longer-wavelength lights at night. Several lighting companies, including GE, are also rolling out household bulbs that change wavelengths as the day goes on.
Go camping for the weekend: A recent study by University of Colorado-Boulder researchers found that as little as 48 hours spent in the woods, with no artificial light at night and as much as 13 times more natural light by day, prompted campers to shift into nighttime physiology (signaled by a rise in melatonin) 1.4 hours earlier. They also fell asleep earlier than others who didn’t go camping and were less groggy Monday morning.