Just as the ear has two purposes — hearing and telling you which way is up — so does the eye. It receives the input necessary for vision, but the retina also houses a network of sensors that detect the rise and fall of daylight. With light, the body sets its internal clock to a 24-hour cycle regulating an estimated 10 percent of our genes.

Light hitting the retina suppresses the production of melatonin — and there lies the rub. In this modern world, our eyes are flooded with light well after dusk, contrary to our evolutionary programming. Scientists are just beginning to understand the potential health consequences. The disruption of circadian cycles may not just be shortchanging our sleep, they have found, but also contributing to a host of diseases.

“Light works as if it’s a drug, except it’s not a drug at all,” said George Brainard, a neurologist at Thomas Jefferson University in Philadelphia and one of the first researchers to study light’s effects on the body’s hormones and circadian rhythms.

Any sort of light can suppress melatonin, but recent experiments have raised novel questions about one type in particular: the blue wavelengths produced by many kinds of energy-efficient light bulbs and electronic gadgets.

Dr. Brainard and other researchers have found that light composed of blue wavelengths slows the release of melatonin with particular effectiveness. Until recently, though, few studies had directly examined how blue-emitting electronics might affect the brain.

So scientists at the University of Basel in Switzerland tried a simple experiment: They asked 13 men to sit before a computer each evening for two weeks before going to bed.

During one week, for five hours every night, the volunteers sat before an old-style fluorescent monitor emitting light composed of several colors from the visible spectrum, though very little blue. Another week, the men sat at screens backlighted by light-emitting diodes, or LEDs. This screen was twice as blue.