You come into contact every day with light-emitting diodes (LEDs) -- they illuminate alarm clocks, new televisions, traffic lights, and smartphone displays. Increasingly, you will see white-light versions of LEDs becoming available for energy-efficient home lighting, car headlights, and streetlamps.
What you may not know is that the most common form of white LEDs -- which emit a spectrum of colors, including blue light -- is inadvertently effective at sending signals to our brain's biological clock, which regulates daily activities such as sleep.
The realization of the body's special sensitivity to blue light has spurred scientific investigations of whether the light can disrupt our circadian rhythms, the roughly 24-hour cycle in animals that sets the body's patterns for sleep and other biological processes.
While organizations such as the International Dark-Sky Association urge caution on using white LEDs for outdoor nighttime lighting and some scientists are already calling for regulations to ban the outdoor use of blue-rich light, others estimate that the effects are small and caution that more rigorous scientific studies are needed before determining if white LED light has any health impacts at all. Some scientists argue that other factors, such as sleep deprivation and abnormalities in a person's overall 24-hour pattern of exposure to light and dark, may do much more to disrupt circadian rhythms. Epidemiological studies have linked circadian disruptions to health problems, such as cancer, cardiovascular disease, and obesity, and scientists are trying to determine if light at night -- and blue-rich LEDs -- are a cause.
Abraham Haim of the University of Haifa in Israel considers white LEDs a form of "light pollution." "What is called 'friendly' environmental illumination is unfriendly," said Haim, who is a chronobiologist, a scientist who studies biological rhythms and cycles in animals. He has conducted studies showing that blue light can disrupt circadian-related hormones in nocturnal animals such as voles, moles and rats.
In the most common design, white LEDs create a mixture of blue and yellow light that the eye sees as white. Other light bulb varieties, including incandescents and compact fluorescents, tend to produce much less blue.
Until the 21st century, scientists only knew of two types of light-sensitive cells in the eye: rods and cones. But in 2001, David Berson from Brown University established that the eyes of mammals contain a third type of cell for absorbing light.
"This has been a very exciting discovery in the whole world of chronobiology and vision research," said Jay Neitz, a professor of ophthalmology at the University of Washington in Seattle. "We always thought rods and cones were responsible for circadian rhythms and then we find there's a particular cell that [sends signals] to the superchiasmatic nucleus, the brain's central clock important for daily biological rhythms."
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.