Wonder how salmon know that it is time to spawn, what tells squirrels and hamsters to get ready to hibernate, or why so many teenagers fall asleep at their desks in school?

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 this season's events and the rhythms behind them.

Fall Events and Rhythms

• School starts in September. For many teenagers this means an early wake-up call and less sleep during the week.
• Hibernating animals, such as hamsters, squirrels, and marmots, start preparing for winter hibernation.
• As temperatures drop, leaves change color from green to spectacular shades of red, orange, and yellow.
• Atlantic salmon head from the ocean back to the streams where they were born to spawn.
• Many flowers, such as chrysanthemums, asters, and cyclamens, bloom.
• The harvest of grapes begins in September in many vineyards. Late fall is a good time to visit vineyards for a taste of their nouveau wines.