Jet-setters and shift workers now sit in front of glaring white lights to readjust their body rhythms and avoid sleep and alertness problems. New experiments condcuted by Harvard University researchers suggest that they would be better off sitting in front of blue lights. The research also contradicts what many scientists believed for years, that the 24-hour biological clock is set by sight alone. Until 1995, dogma held that the intensity of light striking receptors that give humans color vision also adjust the daily cycle that controls sleep, performance, and other physical and behavioral factors. Now, there is conclusive evidence for a second system that dominates the setting of daily rhythms in creatures from bacteria to international travelers, even blind ones. “The visual system in humans is most sensitive to green light,” notes Steven Lockley of Brigham and Women’s Hospital, a Harvard research and teaching affiliate. “But when we exposed 12 healthy young men and women to the same amount of either green or blue light, their 24-hour rhythms shifted twice as much with blue than with green.”