Blue Light is the Light That Matters: Here's Why
(June 2006; updated 11/2008)

Your eyes are for seeing the world, right? True; but they're also used for one other thing: allowing your brain to know whether it's day or night out there! Turns out this is not done with the same parts of your eye you're using to read these words right now. No; instead there's a special part of light, a special wavelength, that your eye senses to know day from night.  Blue light.  Really.

So, if you're going to be careful with light -- getting enough, and enough dark, and all at the right times -- as my essay on light and dark in bipolar disorder suggests.  But the following essay will show that you may have to be most careful with blue light. 

Light receptors in your eye: Rods and Cones -- and one more you've never heard of

Somewhere along the way you probably learned that your retina, the back of your eyeball, has several different kinds of light receptors. Rods are for night vision, and for motion detection at the edge of your vision, but don't see color. Three kinds of cones sense different wavelengths of light: red, blue, and green; when your brain puts all those together, you have color vision (unless you're missing one; then you're "color blind"). Ah, but there's one more light receptor back there on your retina.  It was discovered in 2001, but as important is it is -- you'll see -- surprisingly few people know about it.  I didn't, until a new tool for depression came along that was built just for this particular light receptor. 

This recently discovered receptorBrainard 2001 is there just to tell your brain whether it's light or dark outside. It can do this even when your eyelids are closed! (Your eyelid is moderately transparent: close your eyes and look at the light in the room where you're sitting -- you can tell when you're looking toward, or away from that light, right?) So it's actually quite old, evolutionarily, a holdover from when eyes where first evolving from a patch of skin that could tell light from dark. (If you have trouble with "evolution", try my little essay on that, linked from the bottom of this page; for now, read on. I think this story actually makes the evolution thing more believable, at least to me). 

Interestingly, this light/dark receptor is tuned to sense only blue light. (Why blue? It penetrates water better than other colors, probably; more on that).  Remember that light includes a spectrum of light colors, from red to dark blue, which you can see if you break it up with a prism. In the graph below, that range is represented by the yellow colored area, from very blue (short wavelength) at the left, to very red (long wavelength) at the right. Our eyes can see all these colors, using the cones. But the wavelength peak with the stripes is the only color the light/dark receptors are watching for. "Real" light consists of a broad spectrum, and "full spectrum" light bulbs emit roughly the same range, with a few little spikes; but the light/dark receptors are stimulated by only a very small fraction of all that light.  

The light/dark sensors connect to your biological clock, rather than to your vision center. For some more detail on that, see the essay on light and dark in bipolar disorder; and for more on exactly how the biological clock works (how do you make a clock out of proteins?) and how light affects it, see The Biological Clock, Light, and Lithium.

Using blue light for "light therapy"

As you probably know, there's a version of depression called "Seasonal Affective Disorder" (SAD), or "winter depression". This kind of depression -- which is very common in people with Bipolar II -- can be treated with light.  Plain old light will do, but in some parts of the country in December plain old light is hard to come by (such as where I live, in the winter-rainy/cloudy Willamette Valley of Oregon). So several decades ago researchers studied whether sitting in front of a bank of lights for 30-60 minutes might help reverse the seasonal sag in mood, and found that indeed some people seemed to respond very well. In the most recent study of "light therapy", sitting in front of a box of light every morning during a Canadian winter was as effective as fluoxetine (Prozac).

But the light needed for this, from a suitcase-sized "light box", was hard to get: insurance companies almost never pay for these boxes, which are expensive, usually around $250-300. They are not very portable. 

However, now a very small light box is available for around $140 (e.g. from Costco last winter) that's much more portable, so you can use it where you happen to be in the morning. The key was finding just the right wavelength of light for this purpose, which allowed the box to be much smaller.  I hope you already know, from the story above, what wavelength that is: blue light -- the striped peak in the graph above, which comes from the manufacturer's website (I have no financial connection to that manufacturer, and get no gain from telling you this story. I'm telling it here because I think it's a great story, and because understanding it may help people get an effective treatment they might not have considered otherwise).

This little light box has been tested for effectiveness just as the older bigger light boxes have, and shown to be superiorGlickman to the "control" condition  -- a dim red light, admittedly not the greatest control condition, but this research has been hard to do, coming up with a plausible "placebo" treatment.  A better test would be a comparison of the blue light versus white light, which might be more plausible as a placebo, as the older light boxes emit a white light. That study has been done, and the blue light was superior (not yet published, the manufacturer tells me, as of June 2006). The best test would be a head-to-head study of the little blue one versus one of the older big ones, which we know from years of research are truly better than a placebo. But the blue-light manufacturers have little incentive to do that study (what if theirs was not as good?), so we're not likely to see that research unless one of the manufacturers of the old light box pays for it!  

WARNING: Light box safety not established (risk for "macular degeneration?")
Blue light is also known to be more capable of causing damage to your retina. But hold on, now. How much blue light does it take, to be harmful? That is, how intense does that light need to be? How does that compare to the amount of blue light you're getting while sitting at work under the flourescent lights all day? Or working outdoors? With or without sunglasses? 

In other words, we don't know yet, as best I can tell, whether a little blue light box that emits 400 lux can do harm.  Compare a full-sized light box at 10,000 lux, or full summer sunlight at 100,000 lux.  But eye doctors are telling their patients who are getting macular degeneration, a process of losing retinal cells that leads to substantial loss of vision, to avoid blue light.  For now, in the 2006 paper by Glickman and colleagues, they cite a "hazard analysis" that suggests these lights are safe, but note that the long-term exposure risks have not been studied.  

Clearly any potential risk  is related to how long you're exposed each time you use the light box. So people who respond very well to even a short period of blue light, say 5 minutes, will clearly be getting less exposure than those who have to use a blue light box -- or any light box, for that matter -- for 30 minute or more.  

Who might need to avoid using the blue light box approach? -- that is, who  is at risk of macular degeneration?  Older folks (one site says "over 55" -- whoa, I'll be there soon...); people with diabetes; and people genetically at risk (I presume this means others in the family with macular degeneration).  If you include people who are eventually going to join one of these groups, well that includes just about everybody, doesn't it.  However, all of our treatments carry some risk.  If you're lucky and you respond very well to the light, you may be able to use it for a only short period of time.  If that were the case, you might even be able to reduce your blue-light exposure from all other sources enough to "make room" in your current daily exposure for some deliberate morning use of a little blue light box.  In other words, by using sunglasses when outdoors, and perhaps even by blocking blue light at night (using a yellow lensed pair of glasses indoors at night, when your house lights are on, as odd as that sounds), you might be able to lower your blue light exposure even while adding a light box in the morning -- if it works well enough. You could try one for a while and find out if it really works well first, as the exposure to blue light being discussed here is a long-term risk. 

To learn more about macular degeneration, here's a very well-organized and thorough site about types of light as they affect the risk of this condition, by the founder of a Macular Degeneration support program:  Artificial Lighting and the Blue Light Hazard .  

If you think you might benefit from light therapy, talk with your doctor about it.  To get ready for that discussion, here's an explanation of light therapy that will teach you a lot of other important aspects of the treatment. A link to the blue light box manufacturer appears there.

If you haven't just come from there, may I recommend that you have a look at my page on Bipolar Disorder: Light and Darkness, which looks at important treatment implications of the research above. This describes the other important use of blue light: avoiding it at night! That's part of an interesting idea known informally as "Dark Therapy". 

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