This book is an outlined for the short study (1- to 2-weeks) of chronobiology, a field of science that explores the relationships between time and biological functions. It develops step-by-step the reasoning that leads to the current scientific understanding of biological rhythms. The unit can be inserted into a standard middle or high school biology course. Because the scientific study of biological rhythms begins with data, Chapter 1 provides a brief review of the ways to collect, graph, and interpret data. Chapter 2 introduces some of the cycles in nature, especially those of the human body-from dream cycles to menstruation to body temperature. Chapter 3 explores how these cycles come about and explains the differences between external and internal influences. Chapter 4 explores the internal workings of organisms to determine whether there is a single master source of timing information that synchronizes an organism's many interacting cycles. Chapter 5 discusses the impact of rhythms on society and asks how an understanding of them could bring progress in medicine, work schedules, and everyday life. Chapter 6 offers a brief historical perspective on the study of biological rhythms, and chapter 7 outlines eight activities that demonstrate cycles in chemicals, plants, and animals. Each activity includes an introduction, materials, set-up, procedures, and possible extensions. (KR)