Michael from f.lux: This research is from a very talented group of researchers, but it is unclear if it will translate from nocturnal mice to humans (as others have said). The evidence in humans is mixed, but it either shows no effect or a tendency in the other direction. There is a study in the same issue (Spitschan) that says there is no effect in humans.

1. First, the study does not question the contribution of melanopsin (the blue-cyan opsin that got everyone talking about "blue light") - it asks a more subtle question: when you hold melanopsin stimulation constant, what does the remaining light do and why? Here they are finding whether the cones oppose or boost melanopsin based on color signals. But regardless of how this works in humans, we should still expect bright-enough blue light at night to be stimulating, because of the response due to melanopsin.

2. Holding the melanopic portion of a light constant is not something we usually do. For most lights we have today, the "blue" lights would be considerably dimmer than the "yellow/red" ones if we did this. When we compare lights of equal visible brightness, the yellow ones are known to have less effect on human melatonin suppression [Chellappa 2011].

3. The evidence in humans is mixed, but it actually goes the other direction (saying blue is more stimulating), or there is no clear effect. In the same issue, a study on humans by Spitschan found a negative result on whether or not S-cone contrast has an effect: https://www.cell.com/current-biology/fulltext/S0960-9822(19)...

3b. Other research (in monochromatic and polychromatic light) finds that humans are more sensitive to blue light than melanopsin would suggest. See a list below.

4. We're all still trying to explain how the transition to dusk is blue/purple, while our own lighting doesn't do that. We have built our lighting to be relatively bright, but warm. It is not "natural" to extend the day like we do, but it likely would not help anything to make the lights more blue, unless they were quite a lot dimmer, or used very novel spectra.

Here is a list of references for the evidence +/- blue sensitivity (not melanopsin) in humans:

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 83x S-cone contrast. The lights here are "pink" (which has a lot of blue) and "orange" which has very little. https://www.cell.com/current-biology/fulltext/S0960-9822(19)...

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&c...

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.

It would be interesting if we could find some "truth" to the idea that twilight colors affect human circadian entrainment - it has been a recurrent idea for many years. We finally have the technology to target melanopsin separately from the S-cone (see Spitschan's work for an example).

For the press these results get, you'd be surprised that there has been extremely little research funding for most of these things in the last ten years. In a way, I hope that mixed results like these might help! How light affects us at lower levels, and how different we are from each other is not "solved" at all, so there is still a lot of work to do.