Biological clocks, diurnal rhythms, hibernation, premature aging—these are phenomena that most lay people recognize as realities. Some take them for granted. Others, more philosophically minded, might ponder about primacy in the cycle of rhythms. Is there a first cause—a master clock in ourselves, or in the universe? Must sleep and wakefulness be linked to day and night? Are seasonal disorders a lust for hibernation? Why should darkness make us somnolent when animal cousins wake at night?

Paradoxically, clocks and rhythms have seemed much better developed in plants and animals than in humans, and protagonists of free will may view a weakening of clocks as a human deliverance from the deterministic tyranny of nature. This volume teaches us, however, that clocks are alive and well within us—and that a disregard for their rhythms attracts penalties, from jet lag in travelers to premature cardiovascular death in shift workers.

Scientists and medics have long been familiar with a number of robust rhythms that have practical import for the understanding and practice of medicine. The hormone levels of the pituitary-adrenal axis are much higher on waking than later in the day, such that the diagnosis of Cushing’s disease is made initially by demonstrating absence of this rhythm. Heart attacks and strokes are more frequent in the early hours. Hypertension specialists like myself document diurnal rhythm on a daily basis as we perform ambulatory blood pressure measurements, and talk of dippers and nondippers. And to a student of the sympathetic system, it has been tempting to relate variation in hemodynamic function to the diurnal variation in norepinephrine release that Garret Fitzgerald and I measured in bygone years—when the EU had not yet invented the 48-hour clock, and medical registrars thought nothing of staying up all night to do research after our day job on the wards!...