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CELLS that help pick out day from night have been discovered lurking at the
back of the eye. These special cells use light to help set the body鈥檚 internal
clock, even in blind animals.

The cell 鈥渄oesn鈥檛 care about contrast or colours,鈥� says David Berson of Brown
University in Rhode Island. 鈥淚t鈥檚 almost like the light meter in your camera.鈥�
The finding is striking because the dogma for more than a hundred years was that
there are only two kinds of light-sensing cells in the eye: the rods and the
cones. Some suspected a third kind existed to record light intensity
(New Scientist, 1 September 2001, p 11),
but no one had been able to pin them down.

Now Berson and his colleagues have identified this mysterious photoreceptor
as a type of retinal ganglion cell. These extend long projections into the brain
to transmit information from the eyes. By isolating ganglion cells in rats and
exposing them to light, Berson鈥檚 group showed that a tiny fraction of
specialised ganglia react by sending a signal into the region of the brain that
regulates the circadian clock鈥攖he key to the body鈥檚 sleeping cycle. They
suspect the cells use a unique pigment called melanopsin to do this.

鈥淭his is an important advance,鈥� says Steven Reppert of the University of
Massachusetts Medical School. He adds that these ganglia react to light at the
wavelengths previously predicted for this clock-setting photoreceptor.

Berson says it鈥檚 very likely that these cells play the same role in humans,
and are involved in controlling the release of the sleep-inducing hormone
melatonin. This might mean you could stimulate the ganglion cells with specific
wavelengths of light to help reset people鈥檚 internal clocks and cure problems
like jet lag, he says.