The recently discovered type of cell, called intrinsically photosensitive retinal ganglion cells, are much smaller in number than other light-sensitive cells -- approximately only one of them for every million cones. But they contain a key light-sensitive protein called melanopsin. When light strikes melanopsin, it can trigger the ganglion cells to send signals to the superchiasmatic nucleus, a small brain region that regulates the body's circadian rhythms.
The newly discovered ganglion cells "play the central role" in sending signals from the retina to the brain's circadian system, said Mark Rea, director of the Lighting Research Center at Rensselaer Polytechnic Institute in Troy, N.Y. As it turns out, melanopsin proteins are most sensitive to light in the wavelength range between 440 and 460 nanometers -- in between indigo and blue. Many white LED designs create blue light centered at around 450 nm.
A 2005 study by chronobiologists in Basel, Switzerland showed that human volunteers exposed for two hours to 460 nm light at night experienced greater reductions in melatonin, a hormone regulated by the body's circadian system, than when they were exposed to a roughly yellow-green light with a higher-wavelength of 550 nm. Melatonin, in addition to helping the body maintain a regular 24-hour rhythm of wakefulness and sleep, is an antioxidant compound that has been shown to protect biological molecules such as DNA.
In the Oct. 2011 issue of the Journal of Environmental Management, Haim of the University of Haifa and his co-authors calculate that white LED light can reduce melatonin levels five times more than low-pressure sodium lamps, which produce yellow-orange colored lights often seen in parking lots.
Haim and his co-authors call for a "total ban of the outdoor emission of light at wavelengths shorter than 540 nm" -- and to go back to older low-pressure sodium lamp designs -- "to reduce the effects of decreased melatonin production and circadian rhythm disruption in humans and animals."
They also call for increased consumer awareness and for bulb producers to state the wavelengths of light produced by their bulbs.
However, Rea said that it is important to find out the absolute amount of melatonin reduction, instead of the relative amount, caused by different types of lighting. Five times greater than a small amount may not be great, he said. In addition, he argued, simple calculations based on the spectrum of the light may not accurately determine melatonin reduction. Instead one must take into account other factors, such as the amount, source and duration of exposure to the light, as well as how directly it reaches the observer. "All this stuff matters in terms of predicting what effect you're going to have," he said.
Haim is aware of these factors and his team would like to follow up with controlled studies that account for them. Meanwhile, Rea and his colleagues are pursuing research on this topic from two angles. First, they have developed a headset to measure light reaching the eye in human subjects over a 24-hour period. They call it the Daysimeter. Measuring the light that actually reaches the eye with calibrated instruments, he said, will be a key next step in getting answers on the effect of light on health problems.
In addition, Rea and colleagues have developed a physiological model of how light reaching the retina is converted into nerve signals that reach the circadian system.