Power words (adapted from Access Science: Encyclopedia of Science and Technology Online)

photoreceptors Cells or groups of cells found in both plants and animals that are sensitive to light.

retina The part of a vertebrate eye that is the end of the expansion of the optic nerve and that also hosts photoreceptors.

rods Photoreceptors in the retina of the eye that are sensitive to dim light.

cones Photoreceptors in the retina of the eye that respond to specific colors of light.

melatonin A hormone that helps to regulate the body’s master clock.

circadian rhythm A cycle of changes in the body that follow a 24-hour cycle that usually matches the light-dark cycle of an organism’s surroundings.