Updated July 1st 2019:
The short answer is yes. Timing and intensity especially matter. Blue light exposure at night, when we normally would not be exposed to sunlight, causes circadian disruption though the suppression of melatonin. Circadian disruption has been linked to a variety of conditions including cancer, obesity, type-II diabetes, macular degeneration, depression, and more.
Really, Blue Light? Why?
Our bodies are exquisitely tuned instruments. We evolved over millions of years in perfect sync with the environment, and our bodies reflect it. For example, our lungs and circulatory system know exactly what to do with the air we breathe. Their form and structure evolved so we could utilize the oxygen in the gaseous atmosphere of our planet. Messing with the ratio of oxygen to carbon dioxide to argon and all the other gasses seems like a really bad idea, right? Sure, we can handle a little bit of variation in the oxygen content of air (think high elevations) but stray too far and things go down hill real quick.
Well it turns out that our eyes and our internal clocks (more on circadian clocks in a minute) evolved with the natural light on our planet over millions of years in a similar fashion.
Your Eye is More Than Just a Camera
When most people think about what their eyes do, sight is the first thing that comes to mind. But, in truth, the signals our eyes send our brains are used for far more than just visual sight.
As it turns out, light happens to be the perfect cue to synchronize our biological processes to a 24 hours time window.
A portion of the brain called the suprachiasmatic nucleus (located in the hypothalamus) is considered to be the "central clock" of the body, and helps time almost every single function of our biology. Your body contains peripheral clocks as well, but we'll focus on the central clock for the time being.
Everything that happens in our bodies is rhythmic. So, in order to make sure that the right thing happens at the right time, we use circadian genes to epigenetically turn on and off other genes. Every single cell in your body has a set of circadian genes that communicate with each other and with the master clock. That's trillions of cells in your body, and trillions of clock genes trying to make sure all of the delicate processes of your body are in tune with one another.
Imagine millions of people trying to make appointments, go on dates, and get work done without clocks that were synchronized. It would be chaos, right? Well, think about your body in the same way. And, to sum up the consequences, here's a nice quote from a journal abstract:
"Mounting evidence indicates that disruption of circadian regulation is associated with a wide variety of adverse health consequences, including increased risk for premature death, cancer, metabolic syndrome, cardiovascular dysfunction, immune dysregulation, reproductive problems, mood disorders, and learning deficits."
Serious stuff, right?
Why Blue Light: The Angle of the Sun
So, let's define blue light before we go any further. We've all seen a prism splitting white light into its constituent components and understand that light from the sun or a lightbulb is a combination of these different colored wavelengths. We don't necessarily see the blue or red when it is combined with all the other colors.
The color temperature is a simple way to describe the balance of these frequencies. If you've bought light bulbs before, you know that the wrong temperature can make a big difference.
Because of how the sun changes its angle in the sky over the course of the day, as well as the properties of the atmosphere, the temperature of the sun's light changes throughout the course of the day. For most of the day, the sun is right in the middle of the spectrum around 5000K.
However, right before dawn and right after sunset, due to the scattering of light in the atmosphere, the sky appears a deep blue close to 10,000K.
Within the natural conditions (i.e. no artificial light) that humans evolved under for millions of years, sunrise and sunset would be the ONLY times we were exposed to light with such a high spike in blue content. And, as you'll see below in the section discussing "lux", the intensity of sunlight provides us with a high amount of blue light that is buffered by the red light present in sunlight.
Because biology is rad AF, we evolved to use these lighting cues from the environment as a way to synchronize our cellular clock genes to a natural 24 hour day. The suprachiasmatic nucleus (the master clock) uses blue light, not red light, as a way of resetting itself because it was a reliable signal that spiked twice a day, and was essentially never present at night.
Note that I use the past tense of "was" in the sentence above. Let's take a look at the current conditions we've created on earth that contribute to the disruption of these delicate environmental cues.
Sweet Lightshow, Bro
As seen from space in the picture above, we've drastically altered our light environment in ways that are completely unprecedented from the rest of history. Edison invented the lightbulb in 1880 and from then on it's been a race to cover the world in light.
The stark difference of the night sky during a blackout.
"What about fire?" you might ask. Well, it turns out the color temperature of fire is around 1900K. Hardly any blue in it at all. Take a look at the chart below to get a good idea of the color balance in the common lights we use.
As you can see, the most commonly available bulbs, compact fluorescents and LED's have significant spikes in the blue range. LED screens like the one you're reading this on also produce a high amount of blue light. Also, notice how smooth the spectrum of halogen and incandescent bulbs is, and how that mirrors natural sunlight. Now compare this to the Fluorescent and LED bulbs' large spikes in the blue range. Unfortunately, LED and CFL bulbs are also being touted as more environmentally friendly and widely dispersed. But, already within the last year or two, large lighting companies like Philips have seen the writing on the wall and are working to develop more biologically friendly lighting. LED and fluorescent lights also have significant issues with flicker rate, which has been shown to cause headaches and fatigue.
So, now that we're bathed 24/7 in blue light, let's talk about why this is really a problem.
Blue Light Breaks Your Melatonin Cycle
Melatonin and Cortisol are two hormones that have a major impact on the way you feel and function. Melatonin is a powerful sleep hormone, antioxidant, and chemical messenger that tells your body its time to rest, recharge, and recalibrate. Think of cortisol as its opposite. Cortisol gets you up in the morning, is released during times of stress or exertion, and activates and mobilizes various systems in your body.
Under ideal circumstances, melatonin and cortisol should have an inverse relationship. That means, that when your body releases melatonin at night, cortisol is suppressed. And, in the morning when cortisol is released, melatonin is suppressed. Take a look at the following chart. In a healthy 24 hour cycle, this is what your cortisol/melatonin rhythm should look like.
Melatonin peaks between 10 PM - 2 AM. Cortisol peaks around 8-10 AM.
To be clear, this is in ideal conditions. We are meant to sleep in complete darkness. Study after study shows that we simply get better sleep when were not exposed to excessive light within roughly 2 hours of bedtime, and this goes especially for blue light.
There are two main effects of blue light exposure. Melatonin suppression and circadian phase shifting or offsetting. Let's start with melatonin suppression.
Essentially, because of the amazing interaction of our eye, the suprachiasmatic nucleus, and the sun that I described earlier, when we are exposed to blue light, it tells our central clock to reset, start producing cortisol, and stop producing melatonin.
It turns out that, there's nearly a perfect dose response curve for the suppression of melatonin. Blue/Violet light in the 450 nanometer range suppresses melatonin production by about 80%. Green light in the middle of the range suppresses it by about 20%, while red, orange, yellow light have a negligible effect on melatonin production.
If you’ve been checking out the research links, you'll have probably seen the word “lux.” This is a way of describing the total amount of visible light present and how intense it is. The more intense the light source, the higher the value in lux.
Complete darkness would be zero lux. As you can see above, something like a night without a full moon is only 0.002 lux. Compared to a moonless night, a typical living room light is 2,499,900 percent more intense. No, that number was not a typo.
Combine intensity (lux) with these violet, blue and green ranges (380–570 nm) and you’ve got a potent cocktail. Higher lux values amplify the damaging effects of these nanometer ranges. That’s why — if you go back to the previously cited study — you see a 301 percent increase in melatonin suppression between the 15 lux and 500 lux control groups.
When melatonin is insufficient, the sleep/wake cycle is thrown off balance, cellular chaos ensues, and the potential for disease to manifest grows tremendously. Additionally, the cortisol rhythm is thrown off. What many people perceive as "adrenal fatigue" is actually cortisol disregulation that has its roots not in the adrenals, but in the hypothalamic tract of the brain.
Blue Light Destroys the RPE
I won't go too far into this effect because it gets pretty technical, but here's a brief overview of why those "spikes" we saw in the charts above are a really bad thing for your eyes. The retinal pigment epithelium (RPE) is a portion of your eye that is responsible for making sure you can regenerate and repair the visual pigments and photoreceptors in your eyes, as well as remove damage cells. So, while it isn't directly responsible for your eyesight, it's responsible for keeping the components of your eye that are responsible for eyesight healthy.
The chart below is shows the cytotoxicity level of different wavelengths of light to the RPE and the resulting cell death. As you can see the blue/violet range is much more lethal to these cells. 
This effect has been known about since the 1960's. There is some speculation that the effects of blue light are much less harmful when received as part of full spectrum light, like the sun. When it is isolated or received in spikes, as from LEDs or CFL's the effect is more pronounced. Destruction of the RPE contributes greatly to macular degeneration and destroys our bodies' biggest store of the omega-3 fatty acid DHA.
How Much Is Too Much?
Sorry burners and bassheads, you're not going to like this. It turns out the human body is extremely sensitive to light. Our eyes can detect a single photon in a room of total darkness. And, it turns out that just one night of phase shifted sleep alters your clock genes and negatively impacts genetic expression. This might be a good time to point out that night shift work is actually now classified as a 2A carcinogen by the World Health Organization.
Both the intensity and the spectrum of the light matter. The little blue LED status light on your phone charger, while not very bright, is enough to disrupt melatonin production because the spectrum is extremely blue. A couple super bright incandescent bulbs will disrupt melatonin production similarly because, while the spectrum has less in the blue range, the intensity is much higher.
A Hidden Cause of Modern Epidemics?
Here's the thing that really gets me. We're all part of a big experiment to see how constant light exposure affects our health and there's no control group. Sure, there are pockets of the world with less light exposure at night than others, but for the most part, we have very little control over it except in our own homes. Until this knowledge filters into the mainstream and there's a real economic incentive to widely distribute better technology, not much will change. Who knows how deeply junk lighting is impacting the uptick in things like diabetes, cancer, and autoimmunity. I personally have a feeling it is a serious factor that is rarely considered but more impactful than most people even begin to realize. Fortunately, there are things you can do to mitigate your risk.
What Can I Do?
The number one thing you can do is to stop the 400-550 nm range of light from hitting your eyes at night before you go to bed. It simply wouldn't be there if we were living before the lightbulb was invented, and it's just not doing us any good.
There are three main approaches to this:
- Change your technology to stop emitting blue light.
- Shield yourself from blue light.
- Shift your habits to limit blue light and get more natural sunlight during the day.
With the first approach, this could mean things like:
- Using a blue blocking screen cover for your devices
- Turning on nightshift mode or the hidden red mode (my favorite) on your iphone
- Using screen tinting apps like Iris (check out my article on Iris here, it's better than f.lux)
- Changing your lightbulbs back to warm incandescent bulbs (or at least use them in specific lamps for night-time)
- Get a red or amber-colored nightlight so you don't have to turn on the bathroom light at night
- Use a lightbulb condom or buy some red bulbs and pretend you're a lemur at night
With the second approach, this could mean things like:
- Using blue blocking glasses at night. They come in a variety of styles. An Amazon search yields tons of results.
- I use these at home
- For nighttime use, make sure you actually buy orange or red tinted glasses. There are a number of computer glasses marketed as blue blockers with clear lenses. They block some blue light but not enough to make a difference at night. For really good protection you do need wraparound lenses.
- Fit-over models are available for glasses wearers that don't want to get new lenses.
- Readingglassesetc.com has the widest variety of tints available for prescription lenses and reading glasses. They offer tint ranges that completely block 550 nanometers and below. I like them because they clearly denote the spectral block range of all their tints and are relatively inexpensive.
- Black out your windows. I can't stress this enough. If light is getting in through your windows and its anything but the moon, your sleep quality is suffering. You shouldn't be able to see your hand in front of your face in your room at night. If you can, get some blackout curtains and figure out how to block the leaks.
With the third approach, this is going to be pretty variable depending on your personal habits and preferences and could include things like:
- Turning off most of the lights in your house after sunset and just using what you need to see.
- Getting more natural sunlight during the day (especially on exposed skin)
- Exposing yourself to natural light in the early morning to help reset your circadian clock
- Sleeping with no lights on in the house.
- Don't use your phone or computer within 1-2 hours of bed.
- Limiting nighttime activities outside the house.
Obviously, some people's lives are far too entwined with nightlife or music to make that last point truly feasible; however, I do think its possible for nearly everyone to make some change or another that will impact your light exposure. A good strategy will involve all three of these categories and I encourage you to experiment and see what happens to your mood, energy level, and sleep quality when you lower your blue light exposure for a week.
What Next?
To sum it up, we're living in unprecedented times. While deforestation and air pollution are travesties easy to understand, I believe that the artificial light environment we've created has a far more drastic and acute impact on human health than either of those things. Fast forward to 100 years from now, and we'll be looking at biologically mismatched light sources like we look at smoking. The important thing between now and then is to be aware of the problem, understand how to fix it, and understand that light is an incredibly important component of health. I view light as equally relevant as food, exercise, or sleep because it has equally powerful effects on our biology. From an epigenetic perspective, there's no discrimination between the different inputs that shift genetic expression. I look forward to a time when our lighting technology can be harnessed for the positive effects of light, but until then, I'll be wearing my goofy looking glasses.
