Researchers have traced the light sensing circuitry for a type of “second sight” that is distinct from the conventional visual system and seems to interact directly with the body’s internal clock. The researchers speculate that subtle genetic malfunctions of this machinery might underlie some sleep disorders.In an article published in the February 8, 2002, Science, a research team led by Howard Hughes Medical Institute investigator King-Wai Yau described the circuitry, which consists of a subset of nerve cells that carry visual signals from the eye to the brain. The scientists showed that circadian pacemaker nerve cells almost certainly depend on a different light-sensing pigment, called melanopsin, than the conventional visual system, which relies on rod and cone photoreceptors arrayed across the retina.