How Does Light at Night Affect My Health?

For much of human history, people in much of the world slept in two shifts at night. The “first sleep” started in the early evening, followed by a period of wakefulness, sometimes called “the watch” around 1am, and then a second sleep until dawn. This two-part rest was the normal sleep cycle until the industrial revolution made indoor electrical lighting more common and people began to stay up later.

That change in lighting was enough to profoundly affect our sleep patterns. Now, research into the visibility of stars at night is sounding the alarm that our nights are getting brighter at a pace no one had predicted. The study shows that nighttime brightness has increased about 10% every year over the last decade.

Will this change in nighttime light lead to further changes in human biology? Researchers recently weighed in on this question in a special issue on light pollution in the journal Science. With very little governmental regulation, the increasing brightness of our nighttime stands to significantly impact our health and quality of life, as well as impacting other species including plants, insects and other animal species.

Light researchers John Hanifin and George Brainard at Thomas Jefferson University have been studying the effects of light on human physiology for decades.  Among many other contributions, they were the first to show how the spectrum of light exposure at night suppresses melatonin secretion in healthy men and women. They co-authored the new Science article with two colleagues Karolina Zielinska-Dabkowska from Gdańsk University of Technology in Poland, and Eva Schernhammer from Medical University of Vienna and Harvard Medical School. Their article discusses the ways light impacts health and what people can do to counteract the problem.

Learn more about the latest research on excess nighttime light exposure and how it might impact our sleep and health below.

Why do we need darkness?

The research on this topic has come a long way and continues to grow. We know that a dark room is good for sleep, whereas one with too much light can alter sleep patterns in a way that ultimately affects our health. Too much light exposure at night – outside the home or inside from devices and other light sources – can cause eye strain, disturb circadian rhythms, suppress melatonin and disrupt sleep. Night-shift workers who are exposed to bright light during the usual sleep hours have higher risk of developing cancer. A number of studies have also linked bright light at night with a risk for cardiovascular disease, type 2 diabetes, hypertension, obesity, depression and other disorders. One study during the COVID-19 pandemic, for example, reported that people had a more severe and longer COVID-19 disease course when they were sleep deprived, worked at night or lived in communities with excess light at night.

Now that we know how much our nighttime environments are becoming brighter, we realize that we could begin to see larger effects on our health, especially in places with glaring light displays.

Why does the light at night have such a big effect on our health?

In the last two decades researchers discovered that we have a third kind of photoreceptor cell in our eyes that helps keep our circadian clock on track. Aside from the cones that detect color and movement, and rods that give us night vision, we also have the ipRGC photoreceptors, which are responsible for correcting circadian cycles (which can make us feel jetlag when we travel across time zones). These cells are responsible for signaling when it’s time for our brains to be alert and change heart rate in response to bright light. They also prompt the release of melatonin from the pineal gland during the evening and night, which helps promote sleep.

What’s interesting is that these circadian photoreceptors are most strongly activated by blue light. That means that to these cells, exposure to blue light in the evening or night, can be the equivalent to daylight exposure. When these cells detect a sufficient amount of blue light, they signal to the body that it’s time for middle-of-the-day alertness.

What’s the cause of increasing light at night? Is the problem the LEDs?

LEDs have revolutionized lighting as an energy efficient long-lasting option. They have also become more prevalent in outdoor lighting. The problem is that without alteration, LEDs tend to emit bluer light than other types of lighting. At a time of the day when we’re supposed to wind down and prepare for sleep, the blue light exposure can make people more alert, and more likely to disrupt their sleep and circadian cycles.

Since LEDs have become more common in outdoor lighting, they certainly could be contributing to the excess light at night. But the problem is aggravated by poor lighting design and excessive lighting, like the car dealerships or ballparks with floodlights on at all hours. It is possible to design LEDs that emit less blue wavelengths and have warmer tones. Likewise, it’s possible to design light fixtures that direct light to certain areas and keep it from splashing into residential windows. These are steps that would help reduce the impact from excess evening and nighttime light for all of us.

How can people counter the problem of excess light and protect their health?

Astronauts on the International Space Station see the sun rise every 90 minutes and are known to experience sleep and circadian disruption. To address this problem, we have contributed to the design and testing of a new LED lighting system that changes to warmer-, redder-toned light as crew members prepare for sleep in their quarters. Back here on Earth, people with bright lights that come through their windows at night would benefit from black-out window shades or curtains.

There are higher tech solutions as well, with adaptive lighting that’s designed to automatically change based on time of day. Many cell phones have nighttime settings that turn the screen a sepia color in the evening.

It’s also important to change the outdoor lighting around your home, if you can. Add a motion detector so that the lights turn on only when needed. Lights can also be designed to point in a particular direction so they don’t flood through windows.

Reach more on improving your sleep with 10 tips for better sleep and top sleep mistakes to avoid.

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