Researchers at the University of Virginia have revealed that eyes use light to regulate the biological clock through a mechanism that is different from the ability to see.
The finding demonstrates that light receptor cells in the eye are central to setting the rhythms of the brain's primary timekeeper, the suprachiasmatic nuclei, which regulates activity and rest cycles.
"The finding is significant because it changes our understanding of how light input from the eye can affect activity and sleep patterns," said Susan Doyle, a research scientist at U.Va. and the study's lead investigator.
The finding appears in the current issue of the Proceedings of the National Academy of Sciences.
The researchers discovered that they could reverse the "temporal niche" of mice - meaning that the animals' activity phase could be switched from their normal nocturnality, or night activity, to being diurnal, or day active.
The investigators did this by both reducing the intensity of light given to normal mice, and also creating a mutated line of mice with reduced light sensitivity in their eyes, which rendered them fully active in the day but inactive at night, a complete reversal of the normal activity/rest cycles of mice.
"This suggests that we have discovered an additional mechanism for regulating nocturnity and diurnity that is located in the light input pathways of the eye," Doyle said.
"The significance of this research for humans is that it could ultimately lead to new treatments for sleep disorders, perhaps even eye drops that would target neural pathways to the brain's central timekeeper," Doyle added.
Biological clocks are the body's complex network of internal oscillators that regulate daily activity/rest cycles and other important aspects of physiology, including body temperature, heart rate and food intake.