Friday, August 31, 2018

blue light

According to the Vision Council, over 80 percent of Americans report using digital devices for more than two hours per day, and more than 70 percent of American adults report their children receive more than two hours of screen time per day (1). With the increasing prevalence of LED-backlit digital devices and the increasing time spent on such devices (especially among children), we are exposed to more blue light today than ever before. Thus, we have an emerging interest in the long-term effects of blue light. Here's a rundown of what we know- and don't yet know- about blue light.

As wavelength (measured in nm) decreases, energy increases.
Blue light is on the high-energy end of the visible light spectrum. image source

What is blue light?

Blue light, often referred to as high energy visible (HEV) light, refers to light with wavelengths ranging from 380 to 500 nm (some may define it as wavelengths 400 to 500 nm). Unlike UV light, which is part of the non-visible spectrum, blue light is found at the high-energy end of the visible light spectrum.

Blue light can be divided into two bands: blue-violet light (roughly 415-455 nm) and blue-turquoise light (roughly 465-495 nm). Studies have shown that the segment of blue light that is most potentially harmful to the eye is at 415 nm to 455 nm (23). Short story: blue turquoise is the good stuff, blue violet is the bad stuff. For the long story, continue reading!

Where does blue light come from?
  • The sun. The majority of our HEV light exposure comes from the sun. Twenty-five to thirty percent of its spectral emissions fall within the HEV range.
  • Artificial light sources. We are seeing a shift from incandescent light bulbs to LED (light-emitting diode) and CFL (compact fluorescent lamp) bulbs because they are more energy-efficient. But they also emit more blue light.
    • Nearly 35% of cool LED emissions are in the HEV range, 25% of CFL emissions. Experts estimate that by the year 2020, 90% of all of our light sources will be LED (45).
image source

  • Digital devices with LED displays. This includes laptops, tablets, smart phones, some e-readers, etc.

Is blue light all bad?
No! Blue-turquoise light is vital for regulating our circadian rhythms, which in turn can effect mood, memory, hormonal balance and overall health (6).

In the eye, there are photosensitive retinal ganglion cells (pRGC) that have a peak sensitivity in the blue light range. When these cells detect blue light, they send signals to the suprachiasmatic nucleus (SCN) in the hypothalamus of the brain, as well as other brain centers. The SCN is considered the body's master clock and these blue-light sensitive cells help set the clock (67).  When the pRGCs detect blue light, they are depolarized, which inhibits the release of melatonin, a hormone that plays a role in sleep regulation. Exposure to HEV before bed appears to reduce/delay the secretion of melatonin and thus affect our ability to fall asleep (8).

This disruption has effects that reach beyond just sleep; it has effects on mood, alertness, memory, and hormonal balance (69, 10)Melatonin suppression and lack of adequate sleep has been associated with a greater risk of obesity, elevated blood pressure, diabetes, heart disease and stroke (11, 12). Exposure to high levels of blue light while sleeping may potentially be associated with an increased risk for certain hormone-related cancers (ie: breast, ovarian) (131415).

Is blue light harmful to the eye?
We know that UV radiation causes harm to the eye, as it is absorbed by the structures in the front of the eye (the cornea and lens). HEV light is different because it mostly passes through the front part of eye, so its greatest threat to eye health lies primarily in the back of the eye (the retina). Blue light can potentially damage the light sensitive cells in the retina.

The lens is the transparent structure located behind the colored part of your eye. It absorbs more blue light with age, so young children are the ones at the greatest risk. As we age, the yellowing of the lens reduces the amount of blue light that is transmitted. The risk increases again after cataract surgery, when the natural, yellowed lens is removed and a clear implant is put in its place.
More blue light is absorbed by the lens with age, meaning less of it is transmitted to the retina.
image source

In vitro and animal studies have shown that visible light can cause damage to the layers of the back of the eye that include the photoreceptors (rods and cones) and the retinal pigment epithelium (RPE). When we discuss the dangers of blue light specifically, we're talking primarily about photochemical damage to the back of the eye (the retina). What does that mean exactly?

Light is a form of electromagnetic radiation, which is a form of energy. It is either absorbed or transmitted when it hits the eye. When light in the blue spectrum is absorbed, formation of excited molecules occurs. These excited molecules can induce chemical transformations within them, and they can also cause molecules around them to become chemically reactive as well. This leads to the development of toxic reactive oxygen species (ROS), which can cause oxidative damage to the outer retinal cells (25). Studies have shown that accumulation of a retinal waste product called lipofuscin was found to make cells even more vulnerable to damage from blue light, leading to cell death (16). There's a lot we don't know about age-related macular degeneration (AMD), but oxidative damage is thought to have a significant influence on its development (17).

There is pigment in the part of the retina called the macula that absorbs blue light and also neutralizes ROS, which reduces the risk of damage. This macular pigment is made up of lutein, zeaxanthin, and meso-zeaxanthin. This pigment acts as a protective filter, and it is known to decrease with age. Studies have found low macular pigment levels to be associated with higher risk of age-related macular degeneration (AMD). We definitely need more studies to see if blue light exposure contributes to this pigment decrease (18)

Can filtering blue light make it more comfortable when on digital devices?
Chromatic aberration is the defocused color fringe around text/images. It occurs when the eye is not able to bring all wavelengths of light into focus in the same place. Blue light scatters more, so it is less easily focused. This "visual noise" is believed to contribute to computer vision syndrome, or CVS. Filtering some blue light may enhance vision by decreasing glare and increasing contrast sensitivity, leading to increased visual comfort and improved visual performance (5, 7, 19).

So what does all this mean practically? 
To date, there is no long-term data on the effects of excessive exposure to HEV light on the eye. The studies done on blue light and retinal cell damage have been done in vitro or in animals models; more research is needed to see if long-term blue light exposure does indeed damage the human retina and lead to increased risk of macular degeneration. However, the potential connection between hazardous blue light and retinal damage suggests it is wise to try to limit our exposure over time. We know that the higher the energy, the more likely it is to result in oxidative stress and damage to biological tissue. Is there enough HEV light given off by digital devices to damage our health over time? How much is too much? We don't know. 

Some practical steps we can take to reduce our risk of experiencing the harmful effects of blue light:
  1. Wear sunglasses. As we discussed, the majority of our blue light exposure comes from the sun.
  2. Monitor screen time. This is especially important for young children, since their eyes allow more blue light transmission. Turn off digital devices a couple of hours before bed to avoid the harmful effects on melatonin secretion and circadian rhythms. The American Academy of Pediatrics offers some helpful tips for parents here.
  3. Utilize blue light filters. There are settings on digital devices, filters to place on computer screens, and spectacle lens options to help filter harmful blue light. Discuss lens options with your/your child's eye doctor. We'll break down some lens options in a future blog post. 
  4. Eat leafy green vegetables. Increasing your intake of the macular pigments we discussed (lutein and zeaxanthin) could potentially increase the level of these protective pigments in the retina. Check out my previous post on essential eye nutrients for more info.

CliffsNotes: Blue light is good and bad. We want to reduce our risk for cumulative harm over time by selectively filtering the portion that is potentially hazardous. There's a lot we don't yet know, but we should be prudent with what we do know. Practical tips: wear sunglasses, turn off digital devices 1-2 hours before bedtime, and talk to your eye doctor about possible dietary changes and blue light filtering lens options for your glasses (especially for young children). 

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