The nocturnal worsening of asthma is a very common phenomenon (1); however, our knowledge of the chronobiological characteristics of asthma is far from complete. It is becoming evident that nocturnal bronchospasm is not caused by a single process, but by complex interaction of many factors that amplify the normal circadian fluctuation in airway caliber (2). Both normal and asthmatic individuals manifest the best lung function at approximately 4:00 p.m. and the worst at 4:00 a.m. (Fig. 1). Normal subjects experience about 8% change; asthmatics, however, start with lower lung function and can have a much more dramatic fall in function overnight (2,3). Moreover, circadian variability in airway hyperresponsiveness in response to inhaled histmaine (4), methacholine (5,6), and adenosine monophosphate (AMP) (6), is proved to occur in asthmatic subjects, with exaggerated responses at night (5,6). In addition, Mohiuddin and Martin (7) demonstrated that the late asthmatic response (LAR) occurred in virtually all subjects with mild asthma when challenged in the evening.