Precise measurement of time

Long before the earth supported life, it was subject to marked period icities: the change from day to night and the change in seasons. These periodicities had a profound effect on the evolution of biological organisms; most animals have a circadian rhythm which leads them to need sleep at approximately 24-hour inter vals even if they are confined to an artificial environment with no peri odic changes in lighting. The all-too familiar jet lag that human beings experience when traveling long dis tances east or west demonstrates the