Melanopsin (Opn4) requirement for normal light-induced circadian phase shifting
Without melanopsin, mice struggled to reset their clocks to a change in light.
What this paper showed
Resetting a body clock is something every traveller has attempted. You land somewhere five hours away and your body spends several days insisting it is still at home. The same process happens quietly and constantly as day length shifts through the year. The question this study addressed was which part of the eye does the resetting.
The obvious answer would be the rods and cones, since they are the cells that detect light. But animals with no working rods or cones could still reset their clocks perfectly well, which meant something else was reading the light. Suspicion had fallen on melanopsin and on a thin scattering of retinal cells that make it.
Here mice were bred without the melanopsin gene. Their vision was fine. What they lost was the ability to shift their clock properly in response to a pulse of light. The response was not gone altogether, which told the researchers something useful as well: rods and cones do contribute, and the two systems work together rather than one replacing the other.
Other groups published similar findings at almost the same moment, which is how the field became confident quickly. Together these papers established a third light sensing system in the eye, one that exists to tell the brain the time rather than to help you see, and gave a mechanism for practical advice about morning light, evening screens and jet lag.
