Wonder why a long trip in an airplane causes jet lag, why night-shift workers really are more tired, or what fireflies have taught us about the human body's ability to track time?

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.

Summertime Events and Rhythms

• People often slow down, relax, and take summer vacations. But they might suffer from jet lag if they fly across one or more time zones.
• Long days full of sunlight raise spirits, but in northern climates the very long days may lead to people sleeping less.
• Most animals begin eating more—food is plentiful and grows best in this season.
• Most plants grow more in the summertime, spending additional energy on showy flowers or storing energy in deeper roots. These processes are all made possible by tapping the abundant energy of the summer sun.
• Humpback, fin, and mink whales migrate to the cool waters of New England, while gray whales spend their summers in the northern Pacific from Seattle up to British Columbia and Alaska.