In what may prove to be the end of the 'circadian rhythm' mystery, researchers have discovered that the rod cells in our eyes set our so-called internal clocks. These are responsible for regulating everything from our sleep-wake cycles to our appetites and hormone levels.
It was known that light coming into our brains via our eyes set those clocks, but no one was sure exactly how this happens.
But a Johns Hopkins biologist - working in collaboration with scientists at the University of Southern California and Cornell University-unlocked part of that mystery recently.
Their study found that rod cells - one of three kinds of exquisitely photosensitive cells found in the retina of the eye - are the only ones responsible for "setting" those clocks in low light conditions.
In addition, the study found that rods - which take their name from their cylindrical shape - also contribute (along with cones and other retinal cells) to setting internal clocks in bright light conditions.
The findings are surprising 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," Nature quoted him as saying.
"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," he added.
In addition, the study has possible implications for older people being cared for in nursing homes and hospitals, he said.
"Older adults often lose their rod cells to age, which means that their caretakers would be well advised to regularly and deliberately expose them to bright natural daylight in order to make sure that their natural, biological rhythms remain in sync so their sleep-wake cycles remain accurately set," said Hattar.
"Otherwise, they could have sleep disturbances, such as intermittent waking or difficulty falling asleep, not to mention the impact on their appetite and other bodily functions," he added.
The study appeared in a recent issue of Nature Neuroscience.