A mouse study has revealed the role of a light-sensitive pigment in sleep regulation, and could deepen our understanding of how color balance — and light-emitting devices in general — affect human sleep cycles. Researchers from Oxford University’s Sleep and Circadian Neuroscience Institute exposed mice to bright light during the night, aiming to disambiguate two physically incompatible responses that had been previously observed — the inducement of sleep and a simultaneous increase in corticosterone, a stress hormone produced by the adrenal gland that causes arousal, or wakefulness. The team sought to understand how the two effects were related, as well as how they’re linked to the blue light-sensitive pigment melanopsin, known to play a key role in human circadian rhythms. The team exposed mice to three different wavelengths of light — violet, blue and green — at night. Based on the existing data about the role of melanopsin in sleep, they expected blue light would induce sleep fastest, as its 470-nm wavelength was closest to the peak sensitivity of the pigment, around 480 nm. However, it was green light that produced the most rapid sleep onset, between 1 and 3 minutes. Blue and violet light delayed sleep — onset took 16 to 19 minutes for blue and 5 to 10 minutes for violet. The researchers confirmed the effect by testing mice using green and blue light at a time when they would usually be less active. To investigate the role of melanopsin, the team performed the same test on mice lacking the pigment. For these mice, the colors had opposite effects: Blue caused rapid sleep onset, while green and violet significantly delayed sleep, showing that melanopsin is necessary for the substantial wavelength-dependent effects of light on sleep. “This study shows that there are different pathways from the eye to the brain — one directly regulating sleep and the other increasing arousal,” said researcher Dr. Stuart Peirson. “Melanopsin has a more complex role than previously thought, affecting both pathways. This is the first time that it has been shown to regulate adrenal stress responses.” Peirson noted that an obvious caveat of the study is that mice are a nocturnal species. As such, green light may be expected to increase wakefulness rather than induce sleep in humans. He also predicted that blue light would further enhance the wake-promoting effects of light by elevating adrenal stress hormones. Sounds like “lights out” is the way to go for a stress-free night’s sleep.