This research was published in Nature Human Behavior.
As you threw open the curtains this morning to revel in the rays of sunlight, a chain reaction of chemical reactions ensured that your biology kept up with the never-ending cycle of day and night.
A certain spectrum of wavelengths in that daylight – what we might normally perceive as blue – stimulated a type of sensory cell in the back of your eye, signaling to your brain that morning has broken and it’s time to reset your body’s internal clock.
These light-sensitive photoreceptors, known as intrinsically photosensitive retinal ganglion cells (ipRGCs), do not contribute to our true color perception. The neighboring cone cells are in charge of this.
“However, the light-sensitive ganglion cells also receive information from the cones,” says Christine Blume, a chronobiologist at the University of Basel.
“This highlights the question of whether the cones, and thereby the light color, also influence the internal clock.”
Blume led a team of colleagues from the University of Basel in Switzerland and the Max Planck Institute for Biological Cybernetics in Germany in an experiment on the impact perceived colors may have on our daily biological rhythm.
What they observed could have some fascinating implications for how we light up our surroundings, potentially questioning some assumptions about utilizing digital technology at night.
When we should be immersing ourselves in darkness and relaxing, we should avoid equipment that emit a substantial quantity of blue light, such as our cellphones, computer monitors, and tablets.
The argument is sound: the ipRGCs in our eyes respond to short wavelengths of electromagnetic radiation, approximately 490 nanometers in size.
If this was the only wavelength accessible, our short-wavelength-sensitive cones would be firing (while the long- and medium-wavelength cones would be relatively quiet), which would be code for the brain to believe everything was a smurfy shade of blue.
Given that blue light scatters from the sky during the day, it makes natural that our eyes would employ this wavelength to signal the start and end of sleep.
When we’re surrounded by the blue-dominated brightness of fluorescent bulbs and LED pixels, our ipRGCs are just as happy to signal to the circadian pacemaker within our minds that it’s time to play; a deception that some studies says could be detrimental to our health.
Blume, on the other hand, suspected that the way a light’s wavelength mix altered color-reading cones could indicate that there’s more to the phenomenon than meets the eye.
“A study in mice in 2019 suggested that yellowish light has a stronger influence on the internal clock than bluish light,” said Blume.
To resolve whether the way cones perceive a range of wavelengths carries any weight in how the blue-triggered ipRGCs function, Blume and her team recruited eight healthy adult men and eight women in a 23-day-long experiment.
After habituating to a specific bedtime for a week, the volunteers attended three visits to a lab where they were exposed to a constant controlled ‘white’ glow, a bright yellow, or dim blue light for one hour in the evening.
In the lead-up to their typical bedtime, and for up to an hour after, the subjects submitted to a range of tests, including monitoring of their brain waves, heart rates, and salivary hormone levels.
None of the analyses revealed any indication that the perceived color of the light affected the duration or quality of the volunteers’ sleep patterns.
Instead, all three light conditions caused a sleep delay, suggesting light in general has a more complicated impact than previously thought.
That’s not to say ipRGCs aren’t affected by ‘blue’ wavelengths of light. Rather, white light that is packed with blue waves but stimulates cone cells into seeing yellows, reds, or purples could still affect our sleep cycles.
Similarly, light that looks blue but isn’t intense enough to provoke the ipRGCs into functioning might have little influence over our body’s daily rhythms.
Phones of the future may one day allow us to switch into a night mode that we don’t perceive in warmer tones.
“Technologically, it is possible to reduce the short-wavelength proportions even without color adjustment of the display, however this has not yet been implemented in commercial mobile phone displays,” says Blume.
