The team was led by Dr. Stuart Peirson from Oxford’s Sleep and Circadian Neuroscience Institute and they were hoping to come to a better understanding of why exposing mice to bright light caused them two very different and physically incompatible responses.
Dr. Peirson says that when mice were exposed to light during the night, it caused them to fall asleep. However, it also caused increased levels of corticosterone which is a stress hormone that is produced by the adrenal gland. This hormone causes arousal, which leads to staying awake or having a hard time falling to sleep. The team wanted to understand how these two effects could possibly be related and how they were linked to a blue light-sensitive pigment known as melanopsin, which is known to play a very large role in setting our body’s natural clock.
Mice were exposed to three different colors of light: violet, blue and green. It was expected, due to previously recorded data about the role of melanopsin and sleep, that the blue light would cause sleep the fastest because the wavelength of the light (470 nanometres, nm) was closest to the peak sensitivity of the pigment (about 480 nm). Turns out, it was actually the green light that produced the quickest sleep with times as short as 1 to 3 minutes. Blue actually delayed sleep anywhere from 16 to 19 minutes while violet was altered by 5 to 10 minutes.
The results mean that mice that are exposed to blue light sleep quite a bit less than those which were exposed to violet or green. The effect was tested by using both green and blue lights at the same moment when mice would typically be less active.
To take a deeper look at the role of melanopsin, the team attempted the same test on mice that lacked pigment. For these particular mice, the colors gave the opposite results. This shows that melanopsin is crucial for the substantial wavelength-dependent effects of light on sleep.
Researchers also discovered that while exposure to all three colors of light increased corticosterone levels in ordinary mice, blue light caused a much higher increase in levels of the hormone. Mice who did not have melanopsin responded to blue light at a much more reduced rate. When corticosterone effects were blocked, the sleep-delaying effect was reduced, suggesting that the production of the hormone actively inhibits sleep.
Peirson says that the study shows that there are different pathways from the eye to the brain. One directly regulates sleep and the other increases arousal. Melanopsin holds a much more critical role than previously believed, giving it the ability to affect both at the same time. A caveat of the study is that mice are a nocturnal species that are active during the night. Green light is expected to wake humans up instead of cause a more tired feeling. The team is then able to predict that blue light will enhance the wake-promoting effects of light by elevating adrenal stress hormones.
The results are an addition to the current understanding of the effects of light emitting devices on humans. Recent studies have shown that the blue light of devices causes sleep to be delayed. It has been discovered that since there are different pathways in the brain that different colors affect in terms of alertness, it is important that we understand how the overall color balance of artificial light could affect a person’s alertness and their sleep.