Blue light gets a lot of attention, and for good reason. From the first rays of daylight to the televisions and smartphones we use before bed, blue light is everywhere, and it plays a key role in whether (and when) we feel alert.
Here’s what to know about the highly talked about light color, including how it influences sleep.
What is blue light?
Blue light is part of the visible spectrum of light — the rainbow we see when sunlight is refracted through a prism or raindrops after a storm.
What we perceive as different colors is actually derived from the various wavelengths that comprise visible light. Red wavelengths are the longest, while blue wavelengths are the shortest. Other forms of light beyond the visible range include longer wavelengths, such as infrared waves, and shorter wavelengths, such as X-rays.
Even though the combined wavelengths of sunlight appear to us as white, about 25% of those rays are actually composed of blue light. These blue wavelengths cue the process, telling us that it’s daytime. As light enters the eye, it triggers specific photoreceptor cells, known as intrinsically photosensitive retinal ganglion cells, which then produce melanopsin, a photopigment that functions as a “daytime sensor,” explains Michael Grandner, Ph.D., director of the Sleep and Health Research Program at the University of Arizona.
“Melanopsin activates neural pathways that send information to the clock in the suprachiasmatic nucleus [SCN] in the hypothalamus,” says Grandner. The SCN, known as the body’s central clock, then sends a signal to the pineal gland to suppress melatonin, the hormone that prompts us to feel sleepy at night.
“Blue light isn’t primarily something that wakes you up,” he points out. “What it does is trigger the release of a chemical that means you sense daylight. And when your internal clock senses daylight, it tells your melatonin production to hold off.”
This process plays an essential role in keeping our circadian rhythm aligned with a 24-hour cycle. (In fact, studies have shown that without any light and dark cues, our internal clock actually runs on a slightly longer cycle that averages 24 ¼ hours.)
Sources of blue light
As smartphones and other backlit portable tech devices have proliferated over the last couple of decades, so has awareness (and debate) about the various downstream health effects posed by so much screen time, including the impact of the blue light they emit.
However, despite the additional blue light we’re exposed to via our devices, the sun is still our primary source of blue light.
Although a gray day may appear relatively dim, “even the most cloudy, overcast day is probably an order of magnitude brighter than a bright office room,” Grandner says.
This can be measured using photopic lux: One photopic lux is the equivalent of a candle flame that’s one meter away from your eyes.
A sunny day is about 10,000 lux, and a cloudy day is likely between 1,500 and 3,000 lux, he says. By contrast, a brightly lit office is probably in the 300 to 500 lux range, while typical living room light in the evening may be about 100 lux.
Despite those lower lux levels, the effect is still strong because we perceive brightness logarithmically, Grandner explains. “To your eye, the difference between one and 10 in brightness is the same as 10 to 100, 100 to 1,000, or even 10,000 to 100,000, which is like looking straight ahead on a clear day or looking up.”
This means the various types of nighttime light we’re regularly exposed to have more of an effect than you might think.
To start, there’s standard household lighting, including lamps and overhead lights. These used to be lit by traditional incandescent bulbs, which produced light by generating heat and emitted light in the yellow range.
However, these older bulbs have largely been replaced by LED bulbs, which are considered more energy-efficient but emit more blue light. As of 2020, close to half of U.S. households were primarily using LED lighting indoors.
Because LEDs are thinner, they’re also used for backlit digital devices like laptops, smartphones, and flat-screen TVs.
The upshot: We’re being exposed to blue light not just during the day — when its melatonin-suppressing cues are helpful — but in the hours before bedtime as well.
How to help keep blue light from affecting your sleep
When it comes to blue-light exposure, timing is everything. As we shifted from an agrarian society to one where most of our day is spent indoors, we’ve drastically reduced our exposure to outdoor daylight.
The result is a double whammy: A paucity of strong light signals during our days, followed by evenings in a well-lit setting. By changing the timing of when we’re exposed to blue light, we’re now getting its alerting signals at times that conflict with being alert and awake during the day and sleepy at night. As Grandner notes, “We changed our environment without changing our biology.”
Increase daytime light
If you’re concerned about your blue-light exposure, think first about how you spend your days.
You can actually lessen the effects of blue light at night by getting lots of daylight, especially during the morning, Grandner says. This light signal during the day essentially “fills your cup.” This means the message the light sends to your circadian system is strong enough that it’s not diluted by additional blue light exposure at night.
The opposite is also true, with insufficient light during the day providing a weaker signal that’s more easily thrown off by the additional conflicting signal in the evening.
“Light at night tells you that it’s earlier,” explains Grandner. But if your internal clock has already gotten sufficient input during the day, the incorrect information at night has less of an impact.
Pay attention to evening light levels
Next, look at evening light levels in your home, starting with ambient lighting. Even though the illumination from lamps and overhead lights may not appear blue to you, it’s still providing an additional dose of these wavelengths at an inopportune time.
This was reinforced by a recent study in which participants were exposed to bright yellow light, dim blue light, or a control condition of white light in the evening. All three scenarios had similar effects on participants’ melatonin levels and their subjective sleepiness. Because of this, the researchers concluded it was the stimulation from the light itself (which still contained blue wavelengths in all three scenarios that, in turn, activated the retinal ganglion cells) rather than the perceived color of the light that mattered.
By dimming and/or turning off indoor lights in the hours before bedtime, you can decrease the amount of blue light they’re emitting and lessen its impact. You can also swap out your bulbs for ones that emit less blue light, but be aware, as per above, not to rely solely on the color of the bulb to make this determination.
Don’t overlook device use
While the additional blue light you’re getting from using smartphones and other devices can also keep you up at night, it’s not the only way evening tech use affects your sleep.
“What you’re doing on the screen probably matters a lot more than the light itself,” Grandner points out. That’s because your online activities, whether you’re scrolling through the news or checking email, are likely keeping you alert and engaged rather than helping you wind down.
While the ideal solution is to disengage from tech before bed, using a filter (or turning on night-shift mode) can also help. Many devices can be programmed so that they’ll automatically shift to a nighttime mode that filters out the blue wavelengths and makes everything look yellowish. This may be less visually appealing, but that’s not a bad thing if the reduced vibrancy prompts you to sign off sooner.
You can also purchase external filters for phones, laptops, and the like that can block blue wavelengths without changing the colors you see onscreen.
Keep in mind, too, that it’s not just before-bed screen time that matters: One study of young adults found that 40% got back on their devices if they happened to wake during the night — the most unhelpful time for additional blue light exposure.
How to block blue light
Yet another option is donning blue-blocking glasses, which counter the blue light from devices as well as ambient light.
The color of the lenses — anywhere from red to blue — affects both the colors you see and the glasses’ effectiveness at blocking blue light. Studies by Grandner and others have found that red- and orange-tinted lenses are the most effective, blocking at least 98% of blue light wavelengths, followed by brown lenses and then yellow (blue-tinted lenses performed the worst.)
Regardless of the color of the lenses in the glasses, however, “if you look at a piece of paper that has blue on it and you can tell that it’s blue,” says Grandner, “they’re probably not working.”
Keep in mind, too, that even the most effective pair of blue-blocking glasses can only do so much to counteract the effects of tech use on your eyes. In fact, studies have found that wearing blue-blocking glasses doesn’t reduce the eye strain that comes from prolonged staring at the screen. This sustained focus on the screen reduces the number of times you blink per minute, which leads to increased eye dryness and can also cause inflammation.
There’s even a name for this type of eye strain: computer vision syndrome. The recommended fix, according to the American Optometric Association, is what’s called the 20/20/20 rule: Every 20 minutes, you should take a brief (20-second) break from staring at your screen and refocus your eyes on something that’s about 20 feet away.
Special considerations for kids
Blue light’s effects can be even greater for children whose pupils are larger than adults’ and allow in more blue light. Unsurprisingly, in a study that exposed both children and their parents to the same levels of evening light, the suppression of melatonin (which prompts sleepiness) was more pronounced in the juvenile participants.
Because of the increased sensitivity of children’s eyes, even dim light can have an impact: One study found that evening exposure to low levels of light dramatically reduced preschoolers’ melatonin levels. Moreover, even close to an hour after the light source was turned off, melatonin levels in the majority of the kids were still at least 50% below their baseline levels.
The benefits of blue light
Although we may not want additional blue light in the evening, daytime exposure to this part of the spectrum plays an important role in boosting alertness and overall mood. In fact, studies have found that people with seasonal affective disorder, who are frequently treated with bright light therapy (which includes multiple wavelengths, including blue), may also benefit from blue-only lighting.
Moreover, morning light exposure is particularly helpful in cueing us to feel alert: It’s the primary zeitgeber that helps keep our internal clock aligned with the 24-hour cycle of day and night.
Why we’re sensitive to the effects of blue light
The effects of blue light in the morning, which cues people that it’s daytime, make sense when you think about how light affects other species, says Grandner.
“In some mammals, shorter day lengths and darkness trigger hibernation,” he notes. “As humans, we might have some of that vestigial biology, and some people may be more sensitive to that — when winter comes, they want to hibernate.”
This has only been exacerbated by how much time we spend indoors, he adds. “Some of us essentially have seasonal affective disorder all the time because we don’t spend any time outdoors,” Grandner says, “so we’re essentially always in winter.”
