Your eye's third cell type: why does blue light set your body clock?

What are the eye's three kinds of light-sensitive cells?

Cones see colour and detail in daylight. Rods take over in the dark and see in greyscale. Those are the ones you learn about at school, and for more than a century researchers believed they were the only cells in the eye that responded to light.

The third kind was found as late as 2002. A research group at Brown University showed in Science that a small share of the retina's ganglion cells, the nerve cells that carry signals from the eye to the brain, respond to light entirely on their own. They kept doing it even when the researchers blocked every signal from the rods and cones. The same cells were wired directly to the suprachiasmatic nucleus, the area of the brain that acts as the body's master clock.

The cells are called ipRGCs, intrinsically photosensitive retinal ganglion cells. Their light sensitivity comes from melanopsin, a protein first described in 1998 in frog skin and later found in the human eye.

What does the cell do if it cannot see?

It works as a light meter. Rods and cones respond fast and to small changes, so you can see movement and edges. Melanopsin cells respond more slowly and add up light over time. Their question is not what is in front of you, but whether it is day or night.

The answer is used in several places in the body. It sets the internal clock to the day, controls how much the pupil constricts, and affects when the pineal gland releases melatonin, the hormone that marks the start of the night. How evening screen light affects melatonin specifically is covered in Blue light and melatonin.

Why is it blue light that counts?

Melanopsin is most sensitive to short-wavelength light. In the 2002 study, the cells' peak sensitivity sat around 480 nm, in the blue. That matches what researchers at Thomas Jefferson University found in 2001, when they tested 72 people with single wavelengths across 627 sessions: light between 446 and 477 nm was the most effective at switching melatonin off.

It makes sense from nature's point of view. Daylight, especially at midday, contains a lot of blue. For almost all of human history, light after sunset contained very little. The cell reads the colour of light as a measure of time.

How do we know the cell exists in humans?

The clearest evidence came in 2007, in a study in Current Biology. Researchers examined two totally blind people without working rods and cones. Even so, their melatonin dropped when they were exposed to light, and they responded most strongly to blue light around 480 nm. One of them could even tell when a blue light was switched on, without seeing it.

The study showed two things. The internal clock has its own route into the brain that does not go through vision. And that route is tuned to blue.

What does that mean for an evening in front of a screen?

Screens, LED lamps and TVs give off light with a lot of blue, and they are often used long after sunset. Melanopsin cells cannot tell a phone screen at 11 pm from a sky at noon. They only report that blue light is there.

That is why the LOWLIGHT lenses change colour with the time of day. A lens cannot change how the cell works, but it can change which light reaches it. The figures below are each lens's filtration according to the supplier's spectral measurement per lens tint.

All three bands cover 480 nm, where the cell is most sensitive. The red lens goes further and filters part of the green as well. Which lens suits which hour is covered in Yellow, orange or red lens.