The petals of many flowers open at a certain time of day.
Mice are only active nocturnally.
People suffer from jet lag because of a dislocation in these internal timepieces, waking up in the middle of the night ready to start the day. What makes all these biological clocks tick is a question that has puzzled Kay for years and one that came his way almost by accident. As an assistant professor at the Rockefeller University in New York, he worked on a project to discover the plant genes responsible for measuring the direction and duration of light. Plants are actually quite good at measuring light, according to Kay, particularly red light, which has a slightly longer wave length. By measuring sunlight, plants make basic decisions about their lives such as when to germinate and flower.