In 1751, Swedish botanist Carolus Linnaeus came up with the novel idea of using flowers as clocks. Morning glories open their trumpetlike petals around 10 a.m., water lilies at 11 and so on through evening primroses and moonflowers. A full array of these blossoms could indicate the time. It was a whimsical notion. But some 350 years later, scientists are seriously interested in the timekeeping mechanisms of nature. "They're so ubiquitous, they're almost a signature of life," says molecular neuroscientist Russell Foster of Imperial College, London.

From cockroaches to humans, Foster explores these internal clocks in a fascinating new book, "Rhythms of Life," co-authored with British science writer Leon Kreitzman. The authors show how the daily patterns known as circadian rhythms--from the Latin circadiem ("about a day")--influence far more than our sleep. Heart attacks are more common in the morning. Women tend to go into labor in the evening. Severe asthma attacks prevail at night. Although we may jet across time zones, circadian rhythms rule. The book traces the century-long quest to unravel their mechanisms, with some startling outcomes--including the recent discovery that certain genes switch on and off in 24-hour cycles. Even our response to medicines may depend on when we take them.

Nature has devised internal clocks for a simple reason: they aid survival. "The early bird really does get the worm," says Foster--thanks to a silent wake-up call before the last of the wigglers burrow underground at around dawn. A mimosa plant spreads its fernlike leaves during the day to create the maximum surface area for photosynthesis, then folds them up at night to reduce water-vapor loss. It's not a mere response to light. "They do this even when kept in the dark," says biologist Eugene Maurakis of the Science Museum of Virginia.

In humans, the master clock is a tiny clump of cells in the brain called the suprachiasmatic nuclei. The clock is reset daily by signals from a novel type of photoreceptor in the eye that Foster discovered. "The blind rely on it, too, provided their eyes haven't been removed," he says. The result is an orchestrated series of biological events that unfolds in sequence. In the hours before breakfast, the body ramps up digestive enzymes to be ready for the first meal. Temperature and blood pressure rise in preparation for the day's demands. (This jump in blood pressure helps explain the morning increase in heart attacks.) As the day wears on, cells reproduce at set times. Hormones rise and fall--many of them according to a predetermined schedule.

The implications for medicine are profound. By timing treatments to complement daily changes in biochemistry, the authors argue, we can boost efficacy and reduce side effects. In one seminal trial, medical oncologist William Hrushesky of the Dorn V.A. Medical Center in Columbia, S.C., found that by simply reversing the times when he administered two chemotherapeutic drugs, he could extend survival in women with advanced ovarian cancer from 11 percent at five years to 44 percent. In all, says Michael Smolensky, editor of the journal Chronobiology International, more than a dozen ailments can currently benefit from carefully timed treatments. In one recent study, he notes, something as simple as a low-dose aspirin at bedtime reduced the rate of pre-term delivery in pregnant women at risk for hypertension from 14 percent to zero. Aspirin in the morning had little effect. Surprise? Not to Foster and Kreitzman. As they show, timing is everything.