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.