As you ripped aside the curtains to bask in the beams of sunlight this morning, a domino effect of chemical reactions made sure your biology stayed in time with the endless loops of day and night.
Specifically, a precise range of wavelengths within that daylight – what we might ordinarily perceive as blue – triggered a type of sensory cell in the back of your eye, telling your brain morning has broken and it's time to reset your body's internal clock.
Those light-sensitive photoreceptors, called intrinsically photosensitive retinal ganglion cells (ipRGCs), don't contribute to our actual perception of color. That's the job of the nearby cone cells.
"However, the light-sensitive ganglion cells also receive information from the cones," says University of Basel chronobiologist Christine Blume.
"This raises the question of whether the cones, and thereby the light color, also influence the internal clock."
Blume led a team of fellow researchers from the University of Basel in Switzerland and the Max Planck Institute for Biological Cybernetics in Germany on an investigation into the effect perceived colors might have on our daily biological rhythm.
What they discovered could have some interesting ramifications for how we light up our world, potentially challenging some presumptions about using digital technology in the twilight hours.
Modern scientific wisdom advises us to avoid devices that emit a significant amount of blue radiance, such as our smartphones, computer monitors, and tablets, when we ought to be wrapping ourselves in darkness and resting.
There's perfectly sound reasoning for this – the ipRGCs in our eyes react to short wavelengths of electromagnetic radiation, roughly 490 nanometers in size.
If this were the only wavelength available, our short-wavelength-sensitive cones would be firing (while the long-and-medium cones would be relatively quiet), which would be code for the brain to think everything was a simply smurfy shade of blue.
Given blue light scatters from the sky during daylight hours, it makes sense our eyes would use this wavelength as a cue to mark the beginning and end of sleep time.
Flooded with the blue-dominated glow of fluorescent bulbs and LED pixels, our ipRGCs are just as happy to signal to the circadian pacemaker inside our heads that it's play time; a deceit some research suggests could play havoc with our health.
Yet Blume had her suspicions that the way a light's mix of wavelengths influenced the color-reading cones could mean 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," says 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.
This research was published in Nature Human Behavior.