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.

The new research found that the second system of light-sensitive cells, located at the back of the retina, had at least some visual sensitivity, and is the primary control for the pupil’s contraction and dilation response to light. Exposure of the second system to blue light caused a decline in melatonin levels.

The research provides confirmation of the earlier work conducted on sighted subjects. The enhanced understanding of the function of human circadian rhythms is potentially useful. Blue light could be used to help people recover from jet lag or to help people working night shifts experience increased alertness.