The daily variations found in many aspects of physiology are collectively known as circadian rhythm (from "circa" meaning "about" and "dien" meaning "day"). Circadian oscillation in clock gene expression can generate quantitative or functional variations of the molecules directly involved in many physiological functions. This paper reviews the molecular mechanisms of the circadian clock, the tr...

The concept of homeostasis, ie, constancy of the milieu interne, has dominated the teaching, research, and practice of medicine during the 20th century. According to this theory, biologic functions and processes are maintained in relative constancy over time. The emerging concepts of chronobiology, the scientific discipline of biologic rhythm study, and the findings from research in this field ...

Aging is generally associated with weakening of the circadian system. The circadian amplitude is reduced and the circadian acrophase becomes more labile, tending to occur earlier with advancing age. As originally noted by Franz Halberg, similar features are observed in the experimental laboratory after bilateral lesioning of the suprachiasmatic nuclei, suggesting the involvement of clock genes ...

For instance, ICR mice, which were housed under standardized light-dark cycle conditions (lights-on and lights-off at 7:00 and 19:00, respectively) at a room temperature of 24 ± 1oC (range) and a relative humidity of 60 ± 10% (range) and were allowed free access to food and water displayed a circadian rhythm in their leukocyte counts with a peak at noon and a trough at midnight (Fig. 1). Many o...

Advancements in chronopharmaceutics, the traditional goal of pharmaceutics is becoming obsolete. Until now, the emphasis has been on formulations that maintain constant drug levels throughout the day. But we now need to develop more biologically appropriate formulations that take account of variations in bodily functions, such as blood pressure, asthma, arthritis, duodenal ulcer, cancer, diabet...

Chronobiology—the science of daily (circadian), monthly, tidal, and seasonal rhythms—has undergone exponential growth in the past decade, with major discoveries at the molecular and neuroanatomic level. The most ubiquitous rhythms are those linked to the 24-hour day-night cycle. These circadian rhythms manifest themselves at every hierarchical level: from the general population (eg, more traffi...