Of light and banishing SAD

In honor of the holiday (Christmas if you’re a Western Communion Christian, Isaac Newton Day for everyone else), our Beautiful but Evil Space Mistress has a post up about “living in the light”. She mentions some of the more tasteful and tacky Christmas decorations in her neighborhood, but particularly the abundance of light. (Note that all major winter festivals involve light — be it the pagan Julfest, Christian Christmas, or the Jewish Chanukah/Festival of Lights.)

Our BbESM grew up outside Porto, Portugal, with a single 60W incandescent bulb hanging off the ceiling of her room, plus a 30W lampshade — and even that was a luxury by historical standards. In fact, her editor notes that, adjusted for inflation, a given amount of luminosity has gotten a whopping 500,000 times cheaper in the past few centuries. Just in the past few decades alone, we’ve gone from 60W incandescent to 8 W LED for the same luminosity.

Sarah also notes that she suffered from mild SAD (seasonal affective disorder) and hence appreciated the light. Now actually, while incandescents (with their very “reddish” light — not to mention most of their energy output being infrared, i.e., heat) are probably still better than darkness, they do not help a whole lot with SAD except at very high luminosities. Why?

We actually have three types of photoreceptors: rod cells, cone cells in three colors, and ipRGCs (intrinsically photosensitive retinal ganglion cells). The absorption maxima of rod cells (night vision) and cone cells (daytime color vision) are illustrated below:


(Fish and birds have a fourth “color” of cones in the near-ultraviolet region, with an absorption maximum around 370 nm.)

The ipRGC’s task, on the other hand, is not vision per se but the regulation of circadian rhythm. Their pigment, melanopsin, has an absorption maximum around 480nm, in the bluish region. (Mutations in the gene that expresses melanopsin are one cause for SAD.) SAD is a major issue in arctic countries (close to 10% of the population in Finland, for example). The traditional treatment (review article here) involves full-spectrum lamps at high intensity (10,000 lux and more). However, it was recently found that blue-enriched light sources at more modest luminosities of 750 lux — or even narrow-band blue light at just 100 lux — yield equally good results, as they selectively stimulate the ipRGCs.

Merry Christmas, happy belated Chanukah/Festival of Lights, or happy Isaac Newton Day, as applicable!