Illustrations: Alexis Jamet

Elemental Light Week

Does Blue Light From Screens Really Ruin Sleep?

This story is a part of Elemental Light Week, a five-day series on what light does for your body, brain, and well-being.

DDreaded short-wavelength “blue” light from smartphones and other screens is supposedly creating a generation of sleepless zombies, spurring the creation of “night mode” apps that tamp down cold blue light and infuse smartphone screens with warmer yellows and reds. But as with zombies, some things appear different in the scientific light of day.

Studies reveal that light at night, especially blue light, can negatively affect sleep duration and quality. But some scientists say light from smartphones, tablets, computers, and TVs before bedtime is just one ingredient — and quite possibly a relatively minor one — in the recipe of environmental and behavioral monsters that keep people up at night.

The common recommendation to turn off screens two to three hours before bedtime remains good advice for kids and for anyone who struggles to fall asleep. But it fails to take into account the way humans are messing with their biological clocks by spending entire days indoors, in relatively dim lighting, leaving a body with no clue when it’s time to go to sleep.

Get outdoors or sit by a window for at least two hours a day, and you probably don’t have to worry about what you look at in the home at night,” says Mark Rea, PhD, a professor of architecture and cognitive sciences at Rensselaer Polytechnic Institute’s Lighting Research Center. “Your ability to sleep at night [will improve] and your sleepiness during the day will be much dissipated if you follow that simple recipe.”

Not every scientist agrees entirely with that bold statement. But in multiple interviews and reviews of the scientific literature on the topic, it’s clear that screens at night are not as evil as we’ve been led to believe.

Shorter wavelengths of visible light result in cooler colors. Image: NASA

Setting the body’s clock

Humans, like all creatures and even plants, have an internal biological clock attuned to the cycle of night and day. Our circadian rhythm, as it’s called, runs on a roughly 24-hour cycle when properly tuned, though it varies by individual (hence, night owls).

In the course of human evolution, only recently have we been exposed to significant light at night beyond fires in caves or torches in castles. So the circadian rhythm is deeply rooted in long-evolved internal or “endogenous” controls residing in the brain’s hypothalamus, explains Ari Shechter, PhD, an assistant professor in the Center for Behavioral Cardiovascular Health at Columbia University Medical Center. It’s also sensitive to outside factors, called “zeitgebers,” which can change the timing or the expression of the system.

Daylight and darkness are the chief zeitgebers. Darkness triggers the brain’s production of melatonin, a hormone that promotes sleep. Bright light suppresses melatonin production.

But not all light is the same, Shechter explains. In fact, the retina, where light is converted into neural signals at the back of the eye, is particularly sensitive to short-wavelength light that we perceive as blue. The effects are two-fold:

“Exposure to light within this range [at night] can delay the timing of melatonin and reduce its overall secretion,” Shechter says. “This short-wavelength light can also increase our neurocognitive alertness.”

Filtering the effect

Shechter and colleagues tested the effects of blue light at night by having adults who had been diagnosed with insomnia wear amber-tinted glasses, which blocked about 65% of the short-wavelength light, for two hours before bedtime over seven nights. Others followed the same protocol but wore “placebo” glasses that didn’t filter any light. After seven nights, each person switched to the other type of glasses and repeated the experiment. When wearing the amber glasses, people got about 30 minutes more sleep and said they slept better and more soundly compared to when they wore the non-filtering glasses.

But Shechter is cautious about interpreting the findings, detailed in the Journal of Psychiatric Research. While blue light may have a greater effect on the circadian system than other colors in the spectrum, “overall brightness is very important as well,” he says. And because the participants in his study had symptoms of insomnia, the findings don’t indicate whether light-filtering glasses at night would improve sleep for others.

In a similar experiment, people wore glasses that blocked short-wavelength light three hours before bedtime, and they wore sleep and activity monitors 24 hours a day. After two weeks, their nighttime, sleep-inducing melatonin levels were 58% higher than before the experiment began. They were sleeping an extra 24 minutes and reported falling asleep faster and sleeping better, the researchers reported in the journal Ophthalmic and Physiological Optics.

“The most important takeaway is that blue light at nighttime really does decrease sleep quality,” says study leader Lisa Ostrin, PhD, an optometrist and researcher at the University of Houston College of Optometry. However, Ostrin adds, “Our findings were a result of filtering all sources of artificial light, including room lights, street lights, and screens. Filtering blue light from screens is likely beneficial for sleep, but filtering all artificial light will provide the greatest benefit.”

Both studies, however, were small, involving 14 and 22 people, respectively. The bottom line: There’s little evidence that using apps or glasses specifically to filter blue light from your devices at night will offer much benefit.

Less research has been done on the effectiveness of night-mode apps. But in one significant study done at Rensselaer’s Lighting Research Center, scientists tested the iPad’s night-shift mode in young adults, finding that it alone was not sufficient in limiting light’s effect on melatonin levels but that instead, overall screen brightness needs to be turned down.

Still, many experts suggest night mode could help people who have difficulty sleeping.

“It seems that using a night-mode-type setting can help to maintain proper nighttime melatonin secretion,” Shechter says. “But more studies need to be done. A very important point to remember when using night mode, however, is that it is not just the spectral composition of the light (i.e., the wavelength) that influences our circadian system but also the overall brightness of the light. So it is not enough to just turn on the night mode and leave the brightness of the device at a high setting. To try and achieve the best results, you should use the night mode but also lower the brightness to a very low level.”

Wake-up call

Light is like a cup of coffee, says William Killgore, PhD, a psychiatry professor at the University of Arizona.

“Light is not necessarily good or bad in and of itself,” Killgore says. “Like caffeine, it all comes down to when you use it. It can be terrible for your sleep if you’re consuming coffee at 10 o’clock at night, but it may be great for your alertness if you have it in the morning.” Likewise: “You don’t want melatonin in the morning because it makes you drowsy and prepares the brain to sleep.”

In fact, blue light’s effect on the circadian system goes well beyond the sleep-wake cycle. “The circadian rhythm is one of the most powerful influences on human behavior,” Killgore says. “Humans evolved on a planet for millions of years with a 24-hour light/dark cycle, and that’s deeply ingrained in all our cells,” he says.

In a new study published in the February 2020 issue of the journal Neurobiology of Disease, Killgore and colleagues reveal the power of blue light at the right time — in the morning. In a randomized clinical trial of people who had suffered mild traumatic brain injury due to a recent concussion or other head injury, Killgore’s team exposed one group to bright blue light for 30 minutes early each morning. After six weeks, the participants were sleeping and waking earlier, reporting less daytime sleepiness, and improved on tests of brain speed and efficiency, compared with a control group that was exposed to bright amber light. Also, neural connections in parts of the brain that drive alertness and connection were strengthened.

“Daily exposure to blue-wavelength light each morning helps to re-entrain the circadian rhythm so that people get better, more regular sleep,” Killgore says. “This is likely true for everybody.”

Another small study exposed 15 college students to bright light in the morning, either white light or blue-enriched light. “The decline of melatonin levels was significantly greater after the exposure to blue-enriched white light in comparison with warm white light,” the researchers concluded last year in the journal Scientific Reports.

The underappreciated importance of broad daylight

Popular articles and even scientific papers on the effects of light often don’t mention the critical importance of this contrast: the ratio of daytime light intensity and duration to the amount and duration of indoor lighting and screens at night, says Rea, the Lighting Research Center scientist.

“If I have bright light during the day, the impact on my circadian rhythm at night by a given light will be relatively insignificant,” Rea says. “But if I spend all day in a dim room, then that same amount of light at night can begin to be disruptive to the wake and sleep patterns and digestion and all sorts of other things.”

Among the few studies to look into this, a small one involving 14 participants found that two hours of reading on a 10.1-inch color tablet (the ASUS Transformer Pad TF700) at full brightness in the evening after six and a half hours of exposure to bright light during the day did not affect sleep. “Light exposure during the day could have had a protective effect against screen light in the evening,” says study leader Frida Rångtell, PhD, a sleep researcher formerly at Uppsala University in Sweden who is now starting her own sleep health business. But, Rångtell notes, there’s not enough science for a definitive answer.

“The modern lifestyle is set up so that a combination of factors can disrupt our circadian rhythms,” says Laura Fonken, PhD, a University of Texas at Austin researcher who has studied the effects of light on the circadian system and metabolism.

“Sure, lighting from screens plays a part in this, but other factors also certainly contribute. For example, when you eat, exercise, answer stressful emails … that can all influence endogenous hormonal signals that feed back on your circadian clock. Light is the most important factor for synchronizing your circadian clock, but it is not the only factor that influences circadian rhythms.”

Lux matters

The effect of light on the circadian system depends not on the intrinsic brightness of a source but what reaches the eye. Just glance directly at a light bulb in an otherwise dim room and then look the other way to grasp the difference. This illuminance at the eye is measured in lux — a highly complex measure of the quantity of light.

On a bright, sunny day, your eyes may receive 10,000 lux or more, experts say. Even on a cloudy day, you’ll likely get at least 1,000 lux. Either provides a strong dose of melatonin suppression. By contrast, typical indoor lighting produces somewhere between 100 and 250 lux at the eye while standard office lighting may range up to around 500 lux.

Screens deliver much less, according to data on 20 devices provided to Elemental by Rea at the Lighting Research Center. At full brightness in color, typical screens in your home deliver anywhere from 37.8 lux to less than one lux at normal viewing distances.

Some of Rea’s data:

  • Desktop computers: 0.5 to 37.8 lux
  • Laptop computers: 1.7 to 14.5 lux
  • Tablets: 0.7 to 5.9 lux
  • Smartphones: 0.6 to 2.1 lux
  • TVs: 0.03 to 0.5 lux

A black-and-white e-reader at full brightness likely delivers no more than 15 lux, Rea says, though he doesn’t have test data on e-readers.

For adults, it takes about 350 to 500 lux at the eyes to significantly trigger the circadian system to promote significant daytime wakefulness and, in turn, promote nighttime sleep, Rea says. “You can get 30 lux at night, and it won’t make any difference,” he says. “The circadian system needs to know the difference between light and dark, day and night.”

In a 2014 study led by Mariana Figueiro, Rea’s colleague and director of the Lighting Research Center, people sitting six to nine feet from a 70-inch TV for 90 minutes experienced no significant suppression of melatonin. In another from 2012, two hours of nighttime exposure to a computer screen delivering 30 lux to the eyes had the same lack of effect.

But in a later study also led by Figueiro, evening time spent on smartphones, computers, TVs and other screens did suppress melatonin levels in teens age 15 to 17, by 23% after one hour and 38% after two hours. “Compared to our previous studies, these results suggest that adolescents may be more sensitive to light than other populations,” the researchers concluded.

What you should and should not do

Limiting screen time before bedtime and during the day makes sense for children, according to Stanford Children’s Health and others and not just for the effect on the circadian system. Limiting screen time is good for overall eye health and encourages kids to be more active, “burning through their natural physical energy, which makes it easier to settle down at bedtime,” the center notes.

Yet it’s hard to imagine any amount of research will discourage the use of screens at night among adults. Fonken, the University of Texas researcher, is a case in point. “Even as a circadian researcher who knows that exposure to light at night is not optimal, I still check my smartphone right before going to sleep because of societal pressures to stay connected,” she says. But she advises people who rarely, if ever, get outside to aim to get more true daylight so their internal clock knows what time it is. And in addition to smartphone night apps and overall screen dimming, she suggests considering smart light bulbs that allow you to shift room lighting to warmer hues at night.

Or, if you’re a lighting Luddite, simply dial down the dimmer switch or turn some room lights off while you’re absorbed in your favorite zombie movie or… wait, be careful what you allow on your screens late at night.

Rea recalls not being able to sleep after his alma mater, Ohio State, lost to Clemson 29–23 in the nighttime Fiesta Bowl in December, owing to what many analysts agreed were a series of bad calls by the refs. “I was upset,” he says. “It wasn’t the light. It was all those bad calls.” On reflection, he notes the potential positive flipside of a little screen time at night: Reading something soothing or even boring can be soporific. “If it’s a bad enough thesis, I can go to sleep right away.”

Explainer of things, independent health and science journalist, author, former editor-in-chief of LiveScience and Space dot com.

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