Virtually every smartphone on the market now has a “night mode” that shifts the display colors toward the warmer end of the spectrum. The idea, based on interesting but far from conclusive research, is that shorter blue wavelengths of light interfere with your sleep cycle. A new study from the University of Manchester suggests that the warmer colors of a night mode display could actually be more disruptive to sleep than an unmodified display.
The rationale for avoiding blue light makes sense on the surface. Inside your eye, there are a special class of neurons called retinal ganglion cells containing a photosensitive protein pigment called melanopsin. The light detected by melanopsin is an important part of regulating the sleep cycle. Absent technology, melanopsin would only detect light during the day, but we have all these electronic doodads that beam light into our eyes at all hours of the day and night. Since melanopsin is more sensitive to shorter wavelengths of light, the logic goes that you can lessen the negative aspects of screens by lowering the level of blue light.
Smartphone displays have blue subpixels, so night mode simply lowers the light output from those. The result is a warmer, yellow image on the screen. It’s an easy fix, which made it a popular feature on smartphones. However, the University of Manchester study looked at a different aspect of light exposure.
Using a mouse model, the researchers discovered evidence that yellow wavelengths of light may have an even more substantial disruptive effect on sleep patterns than blue light. This doesn’t have anything to do with melanopsin, though. Your eye has groups of light-sensitive cells called retinal cones that are responsible for color vision. The researchers speculate that the brain associates yellow-shifted colors with daytime, whereas blue colors are closer to twilight. That effect may outweigh any benefit from lowering the amount of melanopsin activated via blue light, based on observing the test animals.
There are still plenty of unknowns here, but the research on smartphone displays and sleep has never been very compelling. Whatever the wavelength of light emanating from your phone, it’s still light bombarding your retinas from a few inches away. That’s something that didn’t happen until very recently in human history. It’s not crazy to think that it could affect your sleep.