Circadian rhythms govern our state of arousal, informing us when to go to sleep and (for some) pulling us up from the pleasant depths of slumber. Further, because the physiological goals of sleep are so different from wakefulness (learning while awake & consolidating memories while asleep, using muscles to do work during the day & repairing them at night, et cetera), the circadian rhythm is also used to regulate many bodily properties and dynamics. An example of this can be found in a recent paper published in the journal Cell1. The authors of this study found that the degree of electrical coupling between rod cells and cone cells in mice and goldfish is modulated by circadian rhythms.
Rod cells are relatively color-insensitive cells in the eye while cone cells are color-sensitive (with different subtypes having being sensitive to different parts of the spectrum). Thus, for the purposes of accurate perception of the visual world, one wouldn’t normally want to link the activity of rods and cones because this would mix the color-insensitive responses with the color-sensitive ones, effectively washing out color information. However, as the day goes on and it gets dark, the argument goes, color matters less, and the paucity of light leads to the strategy of pooling responses across all light-responsive retinal cells. This is what is achieved by such circadian-rhythm-induced electrical coupling, pooling of responses to illumination.
1. Ribelayga C, Cao Y, Mangel SC. The circadian clock in the retina controls rod-cone coupling. Neuron, 59: 790-801, 2008.