Timing is everything, avers teacher (NYU) and science-writer Rose (Omni, Science Digest), especially if you happen to be Aaron Burr, Jack Benny, or a world-class athlete. But all of us follow rhythms set by internal clocks that beat time from microseconds to months to years. A master clock in the brain, behind the retina, responds to light rays and sets the daily (""circadian"") cycles of sleep and wakefulness so annoyingly upset by jet travel. (Rose cautions against long flights late in pregnancy, since it appears that the mother's clock sets the fetal clock in motion.) The timing of birth itself, on the other hand, seems to depend on hormone signals from the fetus that trigger labor. Rose deals with so many physiologic events that to sort them out he groups them by time intervals. Under milliseconds come nerve actions: an eyeblink, a sneeze, awareness of pinprick. Hormone action takes seconds; so does a breath--and, should you lack oxygen, it would take only 30 seconds before you lost consciousness. Blood-clotting may take minutes; ditto, reactions to aspirin or other ingested drugs. The cycles of sleep and dreaming repeat during the night in 90-minute cycles. The longer rhythms of life--puberty, skeletal growth, menopause--involve many interacting systems not all sorted out. Death itself, as mystery-readers know, is followed by an ordered set of changes that the coroner uses to estimate the time of death. (This part of the book, and a section on the folly of body-freezing, are overabundant in macabre detail.) In general, Rose's style is bright and breezy and a bit sensational (we get to relive the Reagan assassination attempt and read about nature's freaks), but undeniably fact-filled--even to the one-liners (""Blood removed from the body clots in about 6 minutes"") sprinkled throughout.