Interesting article I came across that I thought I would share.
 

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.

Read the entire article here.