Circadian Rhythms: These are physical, mental, and behavioral changes that follow a daily cycle. They respond primarily to light and darkness in an organism's environment.
Phytochrome: This is a type of photoreceptor in plants that absorbs light in the red and far-red region of the visible spectrum. It helps regulate plant processes such as seed germination and shade avoidance.
Biological Clocks: These are internal timing devices that are genetically set to regulate various physiological responses for different periods of activity, rest, feeding, etc.