iven our technology, specifically our reliance on alarms and digital calendars to map out our daily activities, it’s easy to overlook the environmental signals that influence our body’s reaction to the rising and setting of the sun. In fact, it’s because of our technological dependence, and perhaps due to the “Hot Tub” episode from “Seinfeld” (since Kramer can’t be taken seriously), that before my junior or senior year in college, I really never thought that a “mental clock” or the circadian rhythm applied to humans.
Yet, as much as we ignore the role these signals play, they influence hormone release, RNA transcription, body temperature and other functions. Disruptions can cause a lot of immediate reactions such as jet lag, but it is also believed that continued disruption of an individual’s mental clock increases his or her risk for certain cancers.
Researchers have put more effort into understanding the influence and mechanisms behind the circadian clock. For instance, it has been long believed that light quantity, or irradiation, was the primary input for the body’s master clock - the superchiasmatic nucleus (SCN). As light gets brighter, excitation occurs and cells begin to shift into new patterns that prepare for the coming day. Similarly, as the day grows darker, cell patterns and transcription patterns change, and the body begins to prepare for the coming night.
In mammals, the retino-hypothalamic projection, which is composed of a range of retinal ganglion cells, is what delivers visual input to the SCN. These retinal ganglion cells are photosensitive and can have a role in training the circadian clock when rod and cone photoreceptors are not present.
However, since all photoreceptor types are present in this pathway, University of Manchester researcher Tim Brown and his colleagues used this information to form the hypothesis that light quality (color) is another powerful and reliable influencer of the circadian rhythm.
Brown and his team first began research by determining whether changes in colors associated with the earth’s rotation could provide information about the sun’s angle (when viewed from earth) that would be useful for the internal determination of the time of day. Their results showed significant predictability in changes of brightness and color based on the solar angle.
The next step was proving these changes could be noticed by mammalian visual systems. Using mice, they determined that blue to yellow transitions were more noticeable during twilight hours, meaning color was a factor in discerning night from day.
While it was established that color changes could be detected during twilight hours, and it provided significant information about the time of day, Brown and his team needed to find out if color had a physiological impact on mammals. To do this, they set up an experiment where an artificial sky was displayed to freely moving mice in their cages. When researchers tested irradiance alone, they observed electrophysiological changes; however, when their experiment included color, they found the circadian pattern was more accurately altered. For example, in every case where irradiance was tested alone, the body temperature in mice peaked later than when twilight colors were included.
Brown’s results not only suggest that blue and yellow colors during twilight hours influence the circadian rhythm, but the colors also provide more accurate inputs for the SCN that can quickly and effectively train the body around a diurnal cycle.
The immediate question one might have is whether this applies to nocturnal animals and those with colorblindness. According to Brown, this discovery applies to approximately 90% of mammalian species, which includes both diurnal and nocturnal mammals because they have the capacity for blue-yellow color vision. Therefore, this mechanism is not only found in nocturnal mammals, but it can also work for people with red-green colorblindness (deuteranopia).
Brown also found in his research that clock cells in mice can respond to very momentary color changes, which means that geographic latitudes will not have a significantly negative impact on a mammal’s ability to distinguish time of day.
There is no doubt that the circadian rhythm is loaded with sensitive and perplexing layers for researchers to uncover. While this study highlights another reliable influencer, other recent studies have identified how connected the circadian clock and metabolism are, and furthermore how gut flora also has an influential role. So, for a show about nothing, with quips about exaggerated human events and "yadda yadda yadda," perhaps there is more truth to its humor than we originally thought.
Walmsley, L., Hanna, L., Mouland, J., Martial, F., West, A., Smedley, A., . . . Brown, T. (2015). Colour As a Signal for Entraining the
Mammalian Circadian Clock. PLoS Biol PLoS Biology.
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