Diet, exercise, sleep – all are fundamental to our health, but our relationship with light is not mentioned as much. Now, a massive new study suggests that light-driven disorders can take years off our lives.
Researchers tracked nearly 90,000 people in the UK who spent a week wearing wrist-worn activity devices equipped with light sensors. Then they analyzed their risk of dying over the next eight years. The results were published in Proceedings of the National Academies of Sciences.
The study participants with the brightest nights had a 21% to 34% higher risk of premature death, compared to those who were mostly in the dark between midnight and dawn.
The opposite was true for daytime.
People who enjoyed the brightest days had a 17% to 34% lower mortality risk than those who were in dark environments during the day.
The data underscore that light represents an “emerging risk factor for poor health and longevity,” said Daniel Windred, lead author of the study and a postdoctoral fellow at Flinders University in Australia.
Previous large-scale studies have found similar associations between mortality and light exposure, for example using satellite data and self-reports. However, the British study is the first to directly measure personal light environments around the clock.
“It’s a very powerful study,” says Dr. Charles Czeisler, chief of the Division of Sleep and Circadian Disorders at Brigham and Women’s Hospital and Harvard Medical School.
“We’re not talking about a marginal change. We’re talking about huge increases in risk associated with an easily modifiable risk factor,” he says.
While the study can only show a correlation — not prove causation — the “dose-dependent” response to light was evident even when the researchers controlled for factors such as socioeconomic advantage, income and physical activity.
The findings reflect decades of research indicating that our modern relationship with light can spell disaster for our circadian rhythms – the patterns in our physiology and behavior that fluctuate over the 24-hour cycle – which affect sleep, blood pressure, how we use energy, release hormones and countless other functions .
“We flood the night with light that was never possible before and protect ourselves from light during the day,” says Czeisler.
Here are four takeaways from the research.
Outdoor lights are best
The benefits of having bright days were consistent from early morning to late afternoon.
Windred says it’s not hard to interpret the results: They represent people who spent time outside in daylight.
“There’s a huge jump in intensity between an indoor and an outdoor environment,” he says.
We’re talking orders of magnitude.
In a typical indoor environment, you may be exposed to about 100 to 500 lux (a unit of measurement for light), compared to anywhere from 10,000 to over 100,000 lux depending on conditions and time of day. Even a cloudy day can be well over 1,000 lux.
The central circadian pacemaker in our brain is particularly sensitive to light in the morning, and prioritizing light at that time can make you more alert.
But even if you can’t do that, Windred says you’ll still benefit from outdoor lighting later in the day. “If you come home from work in the afternoon and the sun is shining, it’s still a good time to get light.”
In fact, Czeisler says people tend to underestimate the effects of being outside during dawn and dusk — times when you’re exposed to different wavelengths and intensities of light.
“We think these transitions are probably particularly important,” he says. As long as there is daylight, he encourages people to walk outside, preferably for at least half an hour to 45 minutes.
“It doesn’t have to be all at once,” he says, “this will do wonders for their health.”
Look for the contrasts
You can imagine circadian rhythms as undulating currents, with ups and downs that reflect your body’s changes throughout the dark-light cycle.
Digesting food, repairing organs, re-energizing our brains and clearing out toxins all work better if the circadian system resembles a robust wave.
And light is the most powerful signal. During the day it can enhance our rhythms – and at night suppress or change their timing.
“The study shows that you really need this contrast,” says Laura Fonkena neuroscientist at the University of Texas at Austin, “It’s not just about having too little light during the day or too much light at night.”
In other words: You don’t want your day and night light environments to be comparable. It can easily happen if you spend most of your days in an office, without much natural lighting, she says.
In fact, data from the UK suggests that the damage could be compounded if light causes your circadian rhythms to be misaligned on both fronts.
“We estimate that people with both light days and dark nights can live up to five years longer than people with light nights and dark days, says Windred.
Light can be protective
Bright days can also set you up for a better evening – improving your sleep and protecting you from some of the downsides of artificial light at night.
“We know that exposure to full daylight during the day can actually reduce the sensitivity of our circadian system to light exposure at night,” says Windred.
Studies measuring the effect of nighttime light on the sleep-promoting hormone melatonin support this concept: Study participants who spent their daytime hours in dim light had much greater melatonin suppression when they met light at night. That’s compared to those who were exposed to more light during the day.
Czeisler says this doesn’t mean you’ll be completely impervious to the disruptive effects of light during the evening, especially the blue-enriched light emitted from our devices.
“It sends a direct signal to the brain that says it’s daytime,” he says.
Czeisler’s lab has documented that reading from light-emitting tablets in the hours before bed can “shift your circadian rhythm, making it harder for you to fall asleep, harder for you to wake up, and less likely to go to bed at an earlier hour the next day.”
Keep the lights off in the middle of the night
People who had the lowest chance of dying in the coming years were exposed to hardly any light between about midnight and 6 a.m., the study found.
On the other hand, bright light during the darkness of the night — especially between 2:30 a.m. and 3 a.m. — was associated with the highest risk of mortality.
“It’s the most important time to avoid light,” he says, “it also happens to be the time when the circadian system is most sensitive to light.”
In recent decades, researchers have linked light-driven disturbances to all sorts of health problems—obesity, heart disease, diabetes, cancer, mental illness, and other conditions. Scientists even have is displayed that misalignment of circadian rhythms for relatively short periods of time can disrupt blood pressure and how the body handles glucose.
The risks of working the night shift are well documented, particularly for cardiovascular and metabolic health.
In this latest study, however, Czeisler points out that even when shift workers were excluded from the analysis, the harmful effects of bright light at 3 or 4 a.m. were still “very significant.”
The best time to turn off the lights depends in part on your schedule and chronotype—which is your body’s natural preference for being more of a morning or evening person—Fonken says. But the conclusion is simple: the time when you sleep should be as dark as you can make it.
This story was edited by Jane Greenhalgh
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