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How Daily Light Exposure Impacts Your Risk of Death
This Week’s Research Highlight
Background
Imagine your body as a well-choreographed orchestra, where every instrument represents a different physiological system, all playing in perfect harmony.
Circadian rhythms are the conductor, ensuring each instrument comes in at just the right time. When these rhythms are aligned with the natural day and night cycle, the music flows effortlessly.
But when the conductor is out of sync, the instruments fall out of time. The once-smooth melody becomes disjointed, and over time, the cacophony leads to disease or even death.
In essence, timing is everything. Just as a symphony depends on precise coordination, your health depends on your body’s internal clock keeping its rhythms steady and strong. This is why recent research has shown that sleep timing, on a night-to-night basis, is a stronger predictor of risk of dying than how long one sleeps.
Perhaps the most important regulator of circadian rhythms is light exposure. Light helps synchronize the body’s internal clock (circadian pacemaker) with the external environment. Prior experiments in animals have shown that deliberate exposure to light at night leads to premature death, and we know that individuals who are more likely to be exposed to light during the night (such as shift workers) have a higher risk of dying. But there haven’t been large-scale efforts to capture personal light exposure patterns and link them to mortality data…until now.
A new study examined the association between premature death and light exposure — both nighttime and daytime — in more than 88,000 volunteers. Let’s take a look at what they found, and what we can learn from it.
Tracking light and life
To investigate the long term health effects of patterns of light exposure and circadian rhythm disruption, researchers analyzed data from 88905 participants in the UK Biobank. The UK Biobank is a large research project that has been collecting a massive array of health information from volunteers across the UK, to figure out what makes some people more likely to develop diseases than others.
Participants were equipped with Actigraphs — wrist-worn devices that track light exposure — over a 7-day period. These light sensors generated ~13 million hours of personal light exposure data.
During a mean followup period of eight years, 3750 participants died. Mortality data on participants was retrieved from the National Health Service (NHS), including cause of death. The researchers were able to calculate how personal light exposure and circadian alignment at baseline were associated with subsequent risk of mortality. They also adjusted their analysis to account for age, sex, ethnicity, and sociodemographic and lifestyle characteristics.
Impact of light exposure on risk of death — for better and for worse
For the sake of comparison, the researchers divided participants into four groups, based on both night light exposure and day light exposure: 0-50th percentile being the lowest light, and 90-100 percentile being the brightest light. Let’s start with the effects of nighttime light.
Exposure to brighter light at night, unsurprisingly, was found to be linked to greater mortality risk. Compared to those whose night environment was darkest (0-50th percentile), being exposed to brighter night light was associated with a 15-34% greater risk of dying from any cause — the brighter the light, the greater the risk. Notably, this association was stronger for deaths specifically due to cardiometabolic causes, conferring an increase in risk ranging from 22-46%. When the researchers zeroed in on exact hours, they found that exposure to the brightest light at night between 2:30am and 3:00am could elevate cardiometabolic mortality risk by up to 67%.
You might be wondering how much light we’re actually talking about here. The lowest 50th percentile, and thus those with the darkest night environment, averaged 0.6 lux at night. Lux is a measure of illuminance, which describes how much light falls on a surface (and thus how much we see). Meanwhile, folks closer to the middle, in the 70-90th percentiles, were at an average of about 16 lux. That's darker than most indoor spaces, and probably too dim to read easily, but bright enough to navigate pretty easily. The 90th percentile — those at the highest brightness and mortality risk — were at 100 lux. That's the typical brightness of a living room, so this is basically equivalent to leaving the light on at night.
On the opposite end of the spectrum, individuals with brighter day light had lower all-cause mortality risk. Compared to those in the lowest 50th percentile (meaning least bright light exposure during the day), being exposed to more bright day light was linked to a 10-34% lower risk of dying during the study period. Again, the association here was stronger for death due to cardiometabolic causes, with a reduced mortality risk ranging from 16-39%.
How bright was this daytime light? Well, the lowest 50th percentile were exposed to an average brightness of 426 lux, which is around the brightness that you see within standard indoor environments, including office spaces, classrooms, kitchens, or living rooms. The 90th percentile, meanwhile, was exposed to an average brightness of 3810 lux. That’s roughly the brightness that you would experience during an overcast day outside. So these folks were probably getting some regular sunlight.
How dampened daily rhythms silently threaten your health
We can see the impact of light exposure on our daily rhythms through circadian amplitude. Circadian amplitude refers to the strength or intensity of the body’s daily biological rhythms. It represents how much variation there is between the peaks (high points) and troughs (low points) of these rhythms over a 24-hour period. You could think of it as the magnitude of difference between day and night inside the body.
For someone with high amplitude, their internal cycles are very clear: they feel very alert during their peak active hours, they get very sleepy at night, they experience strong hunger signals at meal times, and they show significant differences in biomarkers between day and night.
Someone with low amplitude, on the other hand, has more subtle variations, meaning they probably feel less alert during the day and less sleepy at night, and overall their body’s rhythms (like body temperature, hormone release, and metabolism) are more blurred.
Here is a rough visualization of the relative strength of various biological markers (hormones, body temperature, etc) between strong and weak circadian amplitude over a 24-hour period. Strong amplitude, represented by the blue line, shows higher peaks during active hours and lower troughs during rest periods, as well as a greater contrast between maximum and minimum points. Meanwhile, the red line, representing weak circadian amplitude, shows smaller differences between peaks and valleys and more subtle transitions between active and rest periods.
So why exactly is this phenomenon a matter of life or death?
Well, we have evolved molecular clocks which align aspects of our physiology with the solar cycle. This helps prepare the body for changes in the environment that are associated with predictable behavioral patterns, like physical activity and eating. When the rhythm is out of sync, you’re likely to see all kinds of physiological issues, especially connected to cardiometabolic health.
Just as one example, the activity of mitochondrial enzymes are regulated by circadian clocks in your cells. This is to ensure that you are burning more energy during the day, when you would normally be taking in and using calories from food in a natural environment. When your diurnal rhythm is derailed, insulin sensitivity patterns are impaired, and your body becomes less efficient at metabolizing sugar. This leads to impaired glucose tolerance, and ultimately to greater risk of developing type 2 diabetes. Similarly, the circadian system regulates blood pressure and heart rate. Low circadian amplitude weakens the natural dips in blood pressure that occur during sleep, leading to elevated nighttime blood pressure (a risk factor for heart attack and stroke).
Fortunately, there are a ton of practical steps that you can undertake to enhance your circadian amplitude, and thus boost your daily performance as well as your long-term health.
Light exposure plays a crucial role in circadian amplitude. Early bright light acts sort of like a reset button, creating a strong peak in alertness and telling your body that it is daytime. When followed by full darkness at night, this day/night contrast strengthens the amplitude of your rhythm.
So during the day, you want to try to get some bright morning sunlight shortly after waking, perhaps for 10-30 minutes. If sunlight is elusive for you, you can try using a light box to simulate (scroll down for a good example). Then, at night, you want to limit your exposure to bright light, as well as blue light, for 2-3 hours before bedtime. Screens typically emit quite a bit of blue light, which is extra alerting, so you’ll want to control your interaction with them. To achieve complete darkness when you are trying to sleep, you should try to use eye masks or blackout curtains (recall that the impact of nighttime light exposure on mortality is strongest for 2-3am).
Importantly, light exposure is just one of several factors that you can manipulate for this purpose. For more ideas, you should check out our How-to Guide to Smart Daily Light.
Random Trivia & Weird News
📬 A Belgian city tried to enlist cats as mail carriers, and it went exactly as you would expect.
In the town of Liège in the 1870s, the Belgian Society for the Elevation of the Domestic Cat was looking for innovative ways to deliver mail in rural areas. This supremely optimistic organization felt that cats, with their innate sense of direction, were well suited to do the job. They came up with a plan to have 37 cats carry letters inside small waterproof pouches tied around their necks. The felines were supposed to be trained to go from one point to another, delivering the letters between villages.
Of course, this didn’t really work. The cats often wandered off, failed to follow any direct routes, or returned home on their own terms. The experiment was rapidly abandoned, and seemingly has not been tried since.
Photo credit: Getty
Podcasts We Loved This Week
- Jay Olshansky: Is radical life extension possible? Via Plain English with Derek Thompson.
- Josh Turknett & Tommy Wood: Can you increase neuroplasticity as you age? Via Better Brain Fitness.
Products We Like
Alaska Northern Lights Northstar Light Therapy Lamp
If you’re not able to get outside, the next best thing is probably a light box, which is designed to mimic the uniquely bright and intense light emitted by the big orange ball in the sky. This is especially important if you struggle with circadian rhythm alignment during the colder months or if you suffer from seasonal affective disorder.
But not all of these lamps are created equal. Experts say that good light boxes need to have a large light surface, with a brightness level of 10,000 lux, and surprisingly few on the market actually meet this criteria. Researchers at the Yale School of Medicine compared samples of various lightboxes, and this was one that met their specifications.
Admittedly, it is pretty pricey, and frankly kind of unwieldy (it’s basically a glowing metal suitcase), but it might be your best bet if you're trying to replicate sunlight.
humanOS Catalog Feature of the Week
How-to Guide for Smart Daily Light
We evolved in the presence of natural daily cycles of light and darkness. But the invention of artificial lighting means that we can now fully control when and how much light we’re exposed to, which has altered this relationship. Today, most of us spend the majority of the day indoors, under comparatively dim artificial lights. Then, after sundown, we are exposed to more bright light, and importantly more blue light due to our digital devices. Consequently, we are getting less bright light during the day and less darkness at night.
This is a problem, because as the study above demonstrates, light sends critical signals to the body, and the intensity and timing of this light matters for both your health and your daily performance. In this guide, we discuss how you can achieve a pattern of natural light and darkness in the modern world by adjusting behavior, modifying your indoor spaces, configuring your devices, and more.
Wishing you the best,
