New study finds that light exposure is key to circadian rhythms that affect health:

Dark nights and bright days predict mortality, a new study finds.

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Ma Ray/Getty Images

Diet, exercise, and sleep are all fundamental to our health, but the relationship with light is rarely mentioned. Now, a major new study suggests that the destruction caused by light can rob us of years of our lives.

Scientists tracked around 90,000 people in the UK who spent a week using wrist-worn activity devices equipped with light sensors. Next, they analyzed the risk of death over the next eight years. The result is Proceedings of the National Academy of Sciences.

Study participants whose nights were the brightest had a 21% to 34% higher risk of premature death compared to those who were in mostly dark areas from midnight until dawn.

During the day it was the opposite.

People who enjoyed the brightest daylight hours had a 17% to 34% lower risk of death than those who were in dimly lit environments during the day.

The data highlights that light is “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 light exposure and mortality using, for example, satellite data or self-report. However, the British study is the first to directly measure an individual's lighting environment over a 24-hour period.

“This is a very powerful study,” he says Dr. Charles Zeisler, Director of the Division of Sleep and Circadian Disorders at Brigham and Women's Hospital and Harvard Medical School.

“We're not talking about small changes; we're talking about significant increases in risk associated with easily changeable risk factors,” he says.

Although this study can only show a correlation and not prove causation, even when researchers controlled for factors such as socioeconomic advantage, income, and physical activity, there was no “dose-dependent” response to light. The reaction was obvious.

The findings suggest that our modern-day relationship with light could wreak havoc on our circadian rhythms (patterns of physiology and behavior that fluctuate around a 24-hour period), which affect our sleep, blood pressure, how we use energy, and the release of hormones. It reflects decades of research showing that There are countless other features.

“We now have a flood of light at night that was previously impossible, and a shield from light during the day,” Zeisler says.

Here are four takeaways from the research.

Outdoor light is best

The benefits of bright daylight were consistent from early morning to late afternoon.

Windred says the results aren't difficult to interpret. They say these represent people who spent time outdoors during the day.

“The intensity is very different between indoor and outdoor environments,” he says.

We are talking about an order of magnitude.

In a typical indoor environment, you can be exposed to approximately 100 to 500 lux (a unit of measurement for light). compared to anywhere Depending on conditions and time of day, from 10,000 lux to over 100,000 lux. Even on cloudy days it can be well over 1,000 lux.

Our brain's central circadian pacemaker is especially sensitive to light in the morning, and prioritizing light at that time may make us more alert.

But even if you can't do that, Windred says you'll still be able to enjoy the benefits of outdoor light later in the day. “Even if you come home from work in the afternoon and the sun is up, it's still a good time to get some light.”

In fact, Ciesler says people tend to underestimate the impact of being outdoors at dawn and dusk, which exposes them to different wavelengths and intensities of light.

“We think these transitions are probably particularly important,” he says. But people are encouraged to get outside, ideally for at least 30 to 45 minutes, as long as there's sunlight.

“You don't have to do it all at once, and this can do wonders for their health,” he says.

look for contrast

Circadian rhythms can be visualized as an undulating flow, with ebbs and flows that reflect the body's changes throughout the dark and light cycles.

Digesting food, repairing organs, replenishing energy for the brain, and removing toxins all work better when your circadian system resembles strong waves.

And light is the most powerful signal. We can strengthen our rhythms during the day and suppress or shift their timing at night.

“This study highlights that this contrast is really needed,” he says laura sparks“It's not just that we have too little light during the day or too much light at night,” says a neuroscientist at the University of Texas at Austin.

In other words, day and night lighting environments do not need to be equivalent. She says this can easily happen if you spend most of your day in an office with little natural light.

In fact, data from the UK suggests that if light disrupts circadian rhythms on both fronts, the harm can be cumulative.

“We estimate that people with bright days and dark nights can live up to five years longer than people with bright nights and dark days,” says Windred.

Light has a protective effect

Bright days mean better nights. You will sleep better and protect yourself from the negative aspects of artificial light at night.

“We now know that exposure to daylight light during the day can actually reduce the sensitivity of the human circadian system to light exposure at night,” says Windred.

Studies that measured the effects of nighttime light on melatonin, a hormone that promotes sleep, support this concept. Study participants who spent the day in a dimly lit environment greater melatonin suppression When you encounter light at night. This is compared to people who were exposed to more light during the day.

Zeisler says this doesn't mean we're completely immune to the destructive effects of light at night, especially the blue-rich light emitted by devices.

“This sends a direct signal to the brain that it's daytime,” he says.

Dr. Zeisler's lab found that reading on a luminescent tablet in the hours before bed “can alter your circadian rhythm, making it harder to fall asleep, harder to wake up, and less able to stay asleep.” has been documented. Go to bed earlier the next day. ”

Please turn off the lights at midnight

The people least likely to die in the next few years were exposed to the least amount of light between about midnight and 6 a.m., the study found.

However, bright light in the middle of the night, especially between 2:30 a.m. and 3 a.m., was associated with the highest risk of death.

“That's the most important time to avoid light. It's also the time when your circadian system is most sensitive to light,” he says.

In recent decades, scientists have linked light damage to all kinds of health problems, including obesity, heart disease, diabetes, cancer, mental illness, and other conditions. Scientists even shown Disruption of circadian rhythm over a relatively short period of time can have a negative impact on blood pressure and physical condition. handle glucose.

The dangers of night shifts, particularly for cardiovascular and metabolic health, are well-documented.

However, in this latest study, even when shift workers were excluded from the analysis, the negative effects of bright light at 3 or 4 a.m. were still “very significant,” Ciesler noted.

The best time to turn off the lights depends, in part, on your schedule and chronotype (your body's natural preference for being a morning person or a night owl), says Vonken. But the conclusion is simple. Sleeping time should be as dark as possible.

This article was edited by Jane Greenhalgh