Blue light keeps night owls going.

A study of nine young men suggests that blue light beats back sleepiness and dampens key physiological changes that normally occur in the late evening. Green-yellow light doesn't have the same effect. Blue light, or white light containing it, therefore, might help evening workers stay alert, the researchers say.

Certain light receptors in the eye affect how the body calibrates its internal clock and orchestrates daily oscillations in body temperature and heart rate. To better understand the role of light's color, or wavelength, in those processes, Christian Cajochen of the Psychiatric University Clinic in Basel, Switzerland, and his colleagues had male volunteers spend an evening and night in a room in which the researchers could control the color of the light.

Each subject participated in three rounds of the experiment, which had the same sequence of light and darkness except for one 2-hour period ending 1.5 hours before bedtime. In separate rounds, that period featured indigo-blue light with a wavelength of 460 nanometers, green-yellow light with a wavelength of 550 nm, or complete darkness. Throughout the experiment, the researchers monitored several indicators of each man's daily sleep-wake cycle.

Volunteers in total darkness during the 2-hour period displayed normal nighttime trends in these indicators, including reduced core-body temperatures, slower heart rates, elevated melatonin concentrations in saliva, and increased sleepiness.

A period of blue light strongly suppressed those changes, while green-yellow light had a minimal effect, the researchers report in the March Journal of Clinical Endocrinology & Metabolism.

The researchers suggest that body temperature and heart rate may be regulated by retinal cells that contain a light-sensitive protein called melanopsin. That protein registers only light of relatively short wavelengths, which includes blue light, and is known to influence some aspects of the sleep-wake cycle.--B.H.