circadianrhythm

This Week

Wakey-wakey!
01 Sep 01

                                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.