All forms of life, from the simplest cells to the most complex organisms, show periodicity in some of their biologic activities and functions. The most common of these rhythmic events are those that we refer to as "circadian" (circa, around; dias, day). In humans, the caliber of both the upper and lower airways shows circadian fluctuation that is amplified in disease states. The caliber of the airways of the tracheobronchial tree decreases at night and increases during the day. In asthmatic persons, the nocturnal decrease is amplified, causing peak dyspnea, wheezing, cough, sneezing, rhinorrhea, and nasal stuffiness to occur between 2:00 and 6:00 A.M. The most effective pharmacologic strategies for the treatment of these symptoms appear to be those timed to provide maximal medication between these hours, when it is needed the most.