A recent study published in Nature Human Behavior evaluated whether variation in light color impacts the human circadian system and sleep quality.
Give me some background first.
Per the research, light has a profound effect on human psychology and behavior due to the retinohypothalamic (RHT) pathway that transmits information from the retina to the circadian pacemaker (which sets the 24-hour circadian rhythm) in the suprachiasmatic nuclei of the hypothalamus.
Neuroendocrine (i.e., evening and nocturnal melatonin suppression) and circadian-phase shifting responses to light are primarily driven via the photopigment melanopsin expressed in intrinsically photosensitive retinal ganglion cells (ipRGCs).
Keep going…
Recent evidence has shown that cones are not necessary for circadian and neuroendocrine responses to light, as some visually blind patients suppress melatonin in response to light.
As a result, under continuous lighting conditions, circadian and neuroendocrine responses to light are predominantly driven by melanopsin—which is activated by short-wavelength light and, once activated, suppresses melatonin synthesis.
Which means …
Meaning that the crucial element in this process is the intensity of the light per wavelength, less so the perceived light color.
However, melanopsin-expressing ipRGCs also receive input from cones and rods, which begs the question: Does light color impact the circadian clock as well?
Gotcha. Now, talk about this study.
This question led a research team from the University of Basel in Switzerland, and the Technical University of Munich in Germany to use calibrated silent-substitution changes in light color along the blue-yellow axis to assess whether color-sensitive cones contribute to the human circadian system and sleep.
Investigators exposed 16 health participants to three different light scenarios (one control and two with intermittent flickering [1 Hz, 30 seconds on-off]) for 1 hour, starting 30 minutes after habitual bedtime.
And what was measured?
The primary outcome variable of the study was the circadian phase shift measured by salivary melatonin onset in the three light conditions.
Of note, the light scenarios were calibrated to have equal melanopsin excitation, allowing researchers to measure the respective stimulation of the cones (i.e., light color).
Give me details on these light conditions.
The three light conditions were as follows:
- Baseline control condition (93.5 photopic lux)
- Intermittently flickering yellow-bright light (123.5 photopic lux)
- Intermittently flickering blue-dim light (67.0 photopic lux)
Findings?
The investigators did not find conclusive evidence for differences between the three lighting conditions in:
- Circadian melatonin phase delays
- Melatonin suppression
- Subjective sleepiness
- Psychomotor vigilance
- Sleep
So how does this translate to everyday life?
The study authors noted that smartphones and other displays with night-shift modes typically change color and reduce melanopic excitation in a yoked fashion.
“Our study provides evidence that any effects seen in night-shift mode may be due to the reduction in melanopic excitation,” the authors stated.
They added, “We therefore encourage users of devices with background-lit displays to make use of built-in software or apps such as f.lux in the evenings and during the night.”
Take home.
These findings indicate there is no conclusive evidence of an effect of silent-substitution changes in light color along the blue-yellow axis at constant melanopic excitation on the human circadian clock, psychomotor vigilance, sleepiness, or sleep.
This suggests that the human circadian clock is relatively insensitive to shifts in light color towards warmer color temperatures at constant melanopic excitation.
Next steps?
Future studies will need to evaluate the effects of calibrated color changes along the blue-yellow axis at different circadian phases, mean light levels, and melanopic excitation levels.
It may also be beneficial for tech companies to use metameric light that allows users to reduce short-wavelength proportions without a change in perceived color to mitigate the deleterious effects of screen use at night.