A simple, clearly written introduction to the rhythms of life--how they operate and how it was discovered that cyclic changes in light, temperature, and hormone levels form a timetable for every organism. In the 1700s a French scientist, de Mairan, noticed that a plant in his study opened its leaves each morning and closed them each night at the same time; when he put it in a completely darkened room, the plant continued on schedule. This was the first recorded observation of circadian rhythms--the inherent rhythms of an organism which keep it in phase with its environment (from Latin circa, about, and dies, day). Practical research on the subject began around World War I; the definitive studies in the 1950s by von Frisch (bees) and Kramer (starlings) found that these animals use internal clocks to direct them to flowers and to their summer and winter homes. According to Dr. E. Weitzman--an expert on circadian rhythms in endocrinology and sleep--the persistence and universality of these rhythms make them the third great principle of biology, after stimulus/response and homeostasis. Weston takes note of human rhythms: memory peaks in the morning and fades as lunchtime approaches; mental skills peak in the early afternoon, physical skills in mid-afternoon; there is a temporary drop in many performance levels after lunch; the peak in cell division, when the body rebuilds itself, is between noon and 9 p.m.; and if all external clues are eliminated, the majority of people run on a 25-hour clock. In the future, he suggests, we will know enough to adapt our life styles to take advantage of circadian rhythms. For the moment, he limits his advice to means of minimizing jet lag and overcoming insomnia. Informative for beginners who've been out of school since the subject became a curriculum staple.