Wonder why you feel sleepy as night approaches, or how squirrels know when to hibernate, or why some flowers open and close their petals depending on the time of day?

Welcome to the world of biological clocks—the internal molecular machines that keep track of time in organisms as diverse as fungi, fruit flies, plants, and mammals. Join us on a brief tour of some of these seasonal events and the rhythms behind them.

Springtime Events and Rhythms

• Birds migrate to their summer breeding grounds from their wintering habitats.
• Squirrels emerge from hibernation.
• Lizards crawl forth from winter dormancy.
• Grasses release pollen during the daytime.