Are we sure





  • This study answers an important question in mice, and we don't think it applies (yet) to humans.

    What's amazing is that there is a study saying that there is no such effect in humans in the very same issue. So read this one too: https://www.cell.com/current-biology/fulltext/S0960-9822(19)31501-5

    Regarding the Mouland study, here are some details:

    1. This study does not dispute the contribution of melanopsin, which is the blue-cyan pigment that started this whole "blue light" thing. The question is what's going on in addition to melanopsin.
    2. This is a demonstration in mice, which have a UV-sensitive S-cone, not in humans. The mice are nocturnal, not active during the day like humans, so you might say they need different cues about dawn and dusk.
    3. Several studies in humans (see below) say that humans tend to be more sensitive to blue even than melanopsin would predict (all of these things are measured at night) - others (including the above by Spitschan) say there is no effect
    4. In the above experiment, the blue stimulus is much dimmer than the orange one, so we don't know if this effect would work at light levels that humans would see as comfortable after dark, and this is the main problem for our own lighting

    Stepping back, first: the reason people talk about "blue light" is because of the discovery of melanopsin in the eye (which is sensitive to cyan).

    The question this paper addresses isn't about melanopsin - instead, it asks if there is an additional response at dusk (when blue > cyan) that says "it's more stimulating" or "it's less stimulating". In other words, does melanopsin explain everything, or are the cones boosting it? (Probably the latter is true.)

    So far in humans, there are mixed reports about whether or not there is such an effect:

    1. The Thapan study from 2001 indicates extra blue-light sensitivity in addition to melanopsin. Lights are seen for a half hour at night. https://doi.org/10.1111/j.1469-7793.2001.t01-1-00261.x
    2. The Spitschan study from this same issue of Current Biology says there is no effect in either direction when comparing S-cone contrast. The lights here are "pink" (which has a lot of blue) and "orange".
    3. The Brainard 2015 study compares 4000k to 17000k lights: at the same "melanopic" level the 17000k lights do a lot more melatonin suppression: https://jdc.jefferson.edu/cgi/viewcontent.cgi?article=1081&context=neurologyfp
    4. There is one important study in humans (Gooley 2010) that says we can be more sensitive to 555nm light after two days in dim light, so that mirrors this study. But this is not exactly comparable to the study cited here due to sensitization: it stands on its own due to the duration of the experiment.

    That said, it's very compelling to think of twilight (the "blue hour") which has a kind of purple hue, as having a beneficial effect on circadian entrainment - why would we ignore it in our lighting schemes? There is a difference between 6 hours of "warm light" at night and 1 hour of twilight as is seen in nature, so it's unclear how to merge the two. Our history with electric lighting doesn't simulate twilight, and it's not clear exactly what it would look like.

    Overall, the other research so far in humans says there is either no effect, or that it goes the other direction. These effects are sort of "boosts" to the overall system that detects day and night, and there is likely something like this going on with humans too. It is important to do more research to find out what's going on exactly.


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