more complex eyes have not lost this function. a special type of cells in the eye senses light for a different purpose than seeing. these cells are called "ganglion" cells. they are located in the retina. they send their information about light to the brain along a different path (the "retinohypothalamic tract"). this information adjusts (synchronizes) the animal's circadian rhythm to nature's light/dark cycle of 24 hours. the system also works for some blind people who cannot see light at all.