Minimizing light at night, maximizing light during the day
The low-hanging fruit for longevity
A new study came out this week in PNAS on how light exposure correlates to mortality rates. It has some interesting results that I want to dive into — and explain why light optimization is the low-hanging fruit for increasing healthspan.
What does the study show?
The researchers had 100,000+ study participants from the UK biobank wear light and accelerometer devices for a week (this was 10+ years ago). They analyzed the data for light exposure during the day and light exposure at night, and looked at when people were active and when they were sleeping. Then they looked at the number of people who died over the next 8+ years, causes of death, and a bunch of different parameters. They adjusted the data for socioeconomic status, employment, smoking, alcohol consumption, shift work, and social activity. Physical activity was factored in based on accelerometer information, and they also used this to calculate sleep duration and timing.
The researchers ran several different models on the data, trying to adjust for things like smoking, alcoholism, and socioeconomic factors that are known to increase mortality. The models all showed similar results, with people in the top 30% of light exposure at night having increased cardiometabolic mortality (e.g. heart attack deaths, diabetes) and increased other-cause mortality (e.g. cancer deaths).
Here’s a simplified view of the results (see Table 2 in the study for more details):
What does this mean? People in the top 30% (the 70%-90% level) of light exposure at night were at a 25% increased relative risk of death from cardiometabolic (heart, diabetes) causes and 15% more likely to die of other causes of mortality. In the top 10% (the 90% to 100% percentile) of light exposure at night, individuals had a whopping 46% increased relative risk of death from cardiometabolic conditions and 30% increased mortality for other causes.
(I’ll explain some of the physiological reasons that light at night causes increased mortality in just a bit.)
The other half of the equation looked at in this study is exposure to bright light during the day. Here are the results for how increasing light exposure during the day correlates to decreased all-cause mortality.
People who were in the top half of light exposure during the daytime - e.g. people who went outside - were at a ~20-40% decreased relative risk of death.
Keep in mind that these results are adjusted for all kinds of socioeconomic factors as well as smoking and alcoholism.
Let’s dive into several of the reasons why exposure to light at night and a lack of light during the day have such an overall large impact on mortality rates.
Light at night:
Throughout human history, light at night came in a soft, yellow glow: firelight, candles, gas lamps. With the advent of electric lights, there was an increase in the accessibility of light at night, but the old-fashioned light bulbs were also a softer, yellow light. In the last 30 to 40 years, there's been an explosion of light in the blue wavelengths from LED light bulbs, televisions, cell phones, tablets, and modified street lights.
Light in the blue wavelengths acts on receptors in the eyes that send a signal to the brain to shut down melatonin production. Until the advent of electric lights (and even more so, LED screens), the only time we were exposed to light in the blue wavelengths was during the day.
Link to Cancer Mortality:
The World Health Organization lists nighttime light exposure associated with night shift work as a probable carcinogen. Right up there with excessive formaldehyde exposure.[ref]
Melatonin, which most people think of as the "sleep hormone," rises at night and falls during the day. This is a circadian rhythm that is maintained in almost all animals and is controlled by light hitting the retina of the eye.
But melatonin is not just a sleep hormone. In addition to circadian signaling, recent research shows that melatonin acts as an antioxidant within cells, helping to combat oxidative stress in our cells at night. While we sleep, our cells go into rest and repair mode, cleaning up the waste from the active period during the day. When cells are exposed to too much oxidative stress, it can lead to DNA damage which can increase the risk of cancer.
One large study found a 79% increase in breast cancer risk for women working the night shift for 20 years.[ref]
Another study looking at the combined data from the Nurses Health Studies found that for women exposed at younger ages to night shift work (light at night) there was a more than doubled risk of breast cancer.[ref] Not all studies show such a large risk, with one study estimating only a 7 – 21% increase in risk.[ref]
One study found a 51% increased risk with higher ambient light at night.[ref] It also found that sleeping longer (thus more melatonin) cut the risk of breast cancer in half.
Closing the shutters at night (shutting out the streetlights) was also associated with a significant decrease in cancer risk.[ref]
Cardiometabolic Links:
Melatonin also plays a role in the circadian rhythm of insulin secretion. The pancreas has melatonin receptors to help insulin get down-regulated by melatonin at night.[ref]
Here are some of the studies linking nighttime light exposure to obesity or dyslipidemia:
A study of 100,000 women in the United Kingdom found that nighttime light exposure was associated with obesity, even after controlling for all confounding factors.[ref]
A smaller study of elderly Japanese people came to the same conclusion: those exposed to light at night had almost twice the risk for dyslipidemia and obesity.[ref]
An animal study found that longer exposure to light (16 hours vs. 12 hours) significantly increased weight without changing the amount of food the animal ate.[ref]
Researchers can reverse obesity in animals by changing the light period.[ref]
Melatonin also modulates obesity by affecting the gut microbiome.[ref]
What about the link to heart-related deaths?
The circadian rhythm of blood pressure is an important part of heart health, and it has long been known that there is an association between nighttime light pollution and increased blood pressure.[ref]
A 2021 study found that nighttime outdoor light (e.g., streetlights, house lights) increased the risk of hospitalization and death from coronary heart disease by about 10%.[ref]
A couple of new studies have shown that the reason light at night increases the risk of heart disease is because dim light at night reduces the amplitude of the day-night cardiovascular rhythms.[ref][ref]
Maximizing Melatonin Production:
There are two factors that control melatonin production at night:
Exposure to light during the day – and –
Absence of light at night.
The first factor - exposure to light during the day - was also shown in the new PNAS study to have a similar effect on all-cause mortality as light at night. Increased exposure to light during the day reduced all causes of mortality quite significantly.
Light during the day suppresses melatonin from the pineal gland, and it also causes melatonin levels to subsequently rise during the night. Bright light exposure during the day then causes more melatonin to be produced at night.[ref]
It’s as simple (and complicated) as getting more light exposure during the day. Going outside more or having very bright, full-spectrum lighting.
Studies that involve taking the participants out camping for a few weeks show that the natural light exposure during the day and the lack of artificial light at night causes a shift in melatonin production within a few days. They also found that people who normally stay up later with electric lights had a greater shift to a much earlier sleep time when exposed only to natural light.[ref] Exposure to bright light (e.g. 5000 lux) during the day significantly increases melatonin production, and exposure at midday shifts the onset of melatonin secretion to earlier in the evening.[ref][ref]
What if you work indoors all day? Do whatever you can to maximize your exposure to bright light - sit near a window, bring in a bright desk lamp, eat your lunch outside, go outside during your break time...
Studies show that workers who are in windowless environments also tend to have poorer sleep quality and altered sleep patterns.[ref] In buildings fitted with bright lights in the morning, sleep quality increased and depression decreased in workers.[ref]
Conclusion
Exposure to bright light during the day significantly reduces all causes of mortality, making it an easy and relatively inexpensive way to improve health and longevity. The flip side - avoiding light at night - is also an excellent way to improve healthspan and lifespan. Avoiding light at night takes a little more effort - changing the habit of watching television in the evening with the bright overhead lights on.
I would encourage all of you to give it a try for a week or two to see the benefits of better sleep and overall well-being.
What would this experiment look like? Shut off bright lights two hours before bedtime and switch on low lamps with orange bulbs — avoiding screens and bright lights. Read a book, have a conversation, take a bath, fold laundry, listen to a podcast. Party like it’s 1900… Sleep in a dark room with room-darkening curtains and all night lights or indicator lights turned off or covered up. It’s amazing how bright some of the little LED indicator lights are — cover them up with black tape.
Give it a few weeks of real effort - maxing sunlight during the day and avoiding light at night. See how you feel, and think about how this equates to a 40% reduction in mortality risk. See if you agree that this is the low hanging fruit for maximizing healthspan.
