Melatonin production, sleep, and "cyan light"; how might this finding be possible?

Question

The BBC News article Cyan colour hidden ingredient in sleep describes research that suggests melatonin levels as measured in saliva could be affected by the presence or absence of cyan color in a displayed screen, even when the color balance was adjusted so that the difference was not visibly perceptible.

The colour cyan - between green and blue - is a hidden factor in encouraging or preventing sleep, according to biologists.

University of Manchester researchers say higher levels of cyan keep people awake, while reducing cyan is associated with helping sleep.

The impact was felt even if colour changes were not visible to the eye. (emphasis added)

The researchers want to produce devices for computer screens and phones that could increase or decrease cyan levels.

Is cyan going to keep you awake?

I am guessing that this even possible because color perception comes through the sensitivity of three (or four?) color channels and so two different spectra can appear to be the same color. An example might be that a green wavelength might appear to be the same color as the mixture of a blue and a yellow wavelength if properly adjusted.

But if the two stimuli are adjusted to have the same or similar mix of signals in the individual's color receptors, how might the melatonin production still be affected?

edit: According to Wikipedia, monochromatic light would have a wavelength in the range of 490 to 520 nm, and if you wanted to create a visual impression of cyan, you'd use a mixture of Green and Blue channels:

Wavelength    490–520 nm
Hex triplet   #00FFFF
sRGBB         (r, g, b)   (0, 255, 255)

Answer 1

Melatonin production is in response to photoreceptors in the eye, in part, the same photoreceptors in rods and cones that process and transmit information about the wavelength of light (eventually) to the visual cortex for vision. However, @uhoh was right to wonder whether there might be a separate receptor input for melatonin production.

There is an additional opsin (photosensitive protein), found in the ganglion cells rather than the rods and cones, that is sufficient for responding to light and causing the production of melatonin by the pineal gland (mediated by the suprachiasmatic nucleus of the hypothalamus). Processing of light signals by the circadian system is quite separate from processing by the visual system. This is discussed briefly in Chapter 47 of Kandel's Principles of Neural Science, but you can read more about it in this review.

The pathways involved in receiving and processing light as an input to the circadian system were discovered relatively recently. Consider that the optic tract was discovered in the 3rd or 4th century BC, and the retinohypothalamictract was discovered in 1971.

Answer 2

I am guessing that this even possible because color perception comes through the sensitivity of three (or four?) color channels and so two different spectra can appear to be the same color.

While this can happen, this is probably not what the article is referring to.

But if the two stimuli are adjusted to have the same or similar mix of signals in the individual's color receptors, how might the melatonin production still be affected?

I would guess that the caveat here is "visible to the naked eye" -- that is, the viewers didn't consciously perceive the cyan color. That doesn't mean that your eye didn't transmit its presence, as vision is the most "processed" sense we have. The screen you're reading this on is made up of RGB pixels on a scale small enough that your brain blends them together to construct an image made of the average color of the pixels.

It's very possible that there could be some cyan colors mixed in right now, but not in high enough concentrations for your brain to average them out into something you consciously "see".

For the experiment it's probably something similar: The cyan was there and picked up by the eyes, but the post-eye processing performed by the brain filtered it out from conscious view. That, however, doesn't mean it can't impact other portions of the brain. You still perceived it, it might still have an effect elsewhere, but your brain just isn't presenting it to you as "important" information.