Researchers discover second light-sensing system in human eye
New research on blind subjects has bolstered evidence that the human eye has two separate light-sensing systems — one that perceives the familiar visual signals that allow us to see and a second, separate system that tells our body when it is day or night.
Researchers have long known that the eye performed both functions but until recent years it had been thought that both vision and the management of the circadian rhythm that tells us when to be sleepy and when to be alert had been done all at once through the retina’s rods and cones that enable us to see.
Beginning in the 1990s, however, research in animals and in healthy human subjects indicated that though vision was handled by the rods and cones, the signals that synchronize our body clock with the sun’s rising and setting are handled through a second system of light-sensitive cells, located at the back of the retina. These cells extend from the back of the eye into the brain’s hypothalamus region, which manages our body’s clock.
