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EYES aren’t just for seeing, they also help set our body clock. And new
evidence for how this happens suggests that our eyes have a previously unknown
type of light-sensitive cell.

Light regulates our natural cycle of sleep and wakefulness, partly by
switching off the brain’s production of the sleep-inducing hormone melatonin
during the day. Debra Skene and her colleagues from the University of Surrey,
Guildford, wanted to find out whether some wavelengths of light were better than
others at suppressing melatonin. They shone light into 22 people’s eyes in the
middle of the night, when melatonin levels are naturally highest, to test the
effectiveness of six different wavelengths. As they increased the intensity of
the light, melatonin levels in the volunteers’ blood dropped.

But Skene’s team was surprised to find that the shortest wavelength, which we
see as dark blue, caused the biggest drop in melatonin. This is the wavelength
which the eye’s rods and cones—the cells that gather light for
vision—are least sensitive to. From this Skene concludes that a third,
unknown type of photoreceptor must be telling the brain when to stop making the
hormone.

The sensitivity of photoreceptors to different wavelengths of light depends
on the pigment they contain. Skene’s finding will help researchers identify the
unknown pigment, says neuroscientist Robert Lucas of Imperial College,
London.

Skene now plans to find out whether blue light is also better at regulating
other aspects of the circadian rhythm, such as alertness. If it is, she reckons
it could be used to relieve jet lag or reset shift workers’ sleep patterns. “The
amount of blue in office or factory lighting could be increased to change
people’s body clocks to adapt to their night shift more quickly,” she says.