Green Does Not Mean Go for Mice

    Anyone who has tried to sleep in with the blinds raised on a sunny morning knows all too well that light affects sleep. Yet, visible light, as detected by the human eye, represents an array of colors of various wavelengths. The evidence is continuing to mount suggesting that particular wavelengths affect sleep and wakefulness states within the brain.

    Now, investigators led by scientists at Oxford University have published results from a recent study that shows blue light wavelengths keep mice awake longer, whereas green light wavelengths put them to sleep more easily. The researchers are hopeful that this new information will not only provide new insight into sleep and sleep disorders but could also have potential implications in future lighting designs.

    For mice, which are mostly nocturnal, light is a sleep-inducer. Previous studies in mice and humans have shown that nonimage-forming light perception occurs, in particular photosensitive cells in the eye and involves a light sensor called melanopsin. Mice without melanopsin (Opn4^–/–) show a delay in their response to falling asleep when exposed to light, pointing to a critical role for melanopsin in sleep regulation.

    The Oxford researchers wanted to investigate this phenomenon further by studying sleep induction in mice exposed to colored light, i.e., the light of different wavelengths. On the basis of the physical properties of melanopsin, which is the most sensitive to blue light, the researchers hypothesized that blue light would be the most potent sleep inducer.

    However, much to their amazement, the scientist found that green light puts mice to sleep quickly, whereas blue light seems to stimulate the mice, although they did fall asleep eventually. Mice lacking melanopsin were oblivious to light color, demonstrating that the protein is directing the differential response.

    “We assessed the effects of light of different wavelengths on behaviourally defined sleep,” the authors wrote. “Here, we show that blue light (470 nm) causes behavioural arousal, elevating corticosterone and delaying sleep onset. By contrast, green light (530 nm) produces rapid sleep induction. Compared to wildtype mice, these responses are altered in melanopsin-deficient mice (Opn4^–/–), resulting in enhanced sleep in response to blue light but delayed sleep induction in response to green or white light.”

    The findings from this study were published recently in PLOS Biology in the article “Melanopsin Regulates Both Sleep-Promoting and Arousal-Promoting Responses to Light.”

    Additionally, the researchers found that both green and blue light elevated levels of the stress hormone corticosterone in the blood of exposed mice compared with mice kept in the dark. However, corticosterone levels in response to blue light were higher than levels in mice exposed to green light. Interestingly, when the researchers gave the mice drugs that blocked the effects of corticosterone, they were able to mitigate the effects of blue light—drugged mice exposed to blue light went to sleep faster than control mice that had received placebos.

    The Oxford team feels this work adds the growing body of data concerning the effect of colored lights on humans. For instance, previous work has shown that exposure to blue light—a predominant component of light emitted by computer and smartphone screens—recapitulates arousal and wakefulness in humans, as it does in mice.

    “Despite the differences between nocturnal and diurnal species, light may play a similar alerting role in mice as has been shown in humans,” the authors remarked. Overall, the authors say their work “shows the extent to which light affects our physiology and has significant implications for the design and use of artificial light sources.”