Research appearing in a recent issue of Nature Neuroscience found that rod cells are the only ones responsible for "setting" internal clocks in low light conditions. What’s more, the study found that rods also contribute (along with cones and other retinal cells) to setting internal clocks in bright light conditions.
These findings are intriguing for several reasons, according to study leader Samer Hattar of the Department of Biology at the Krieger School of Arts and Sciences.
"One is that it had previously been thought that circadian rhythms could only be set at relatively bright light intensities, and that didn’t turn out to be the case," he explained. "And two, we knew going in that rods ‘bleach,’ or become ineffective, when exposed to very bright light, so it was thought that rods couldn’t be involved in setting our clocks at all in intense light. But they are."
In the study, Hattar’s team used a group of mice that were genetically modified to have only rod photoreceptors. The team then exposed the rodents to varying intensities of light, measuring the animals’ responding level of activity by how often they ran on hamster wheels.
The study results indicate that prolonged exposure to dim or low light at night (such as that in homes and office buildings) can influence mammals’ biological clocks and "throw off" their sleep-wake cycles. Hattar suggested that one way people can mitigate this effect is to make sure to get some exposure to bright daylight every day.