The pulmonary clock.

Measurement is fundamental to modern medicine, yet few doctors appreciate the importance of analysing biological data, not only as individual results, but also as a signal related to time. Biological rhythms are predominantly circadian-so called because they have a natural period of approximately 24 hours (circa diem) if allowed to run freely in the absence of external stimuli. They can be further described as diurnal if the peak of the rhythm (acrophase) occurs during daylight and the nadir (bathyphase) is seen during the night. Clinicians are familiar with the rhythm in adrenal cortical function,' but many other rhythms-for example, in body temperature,2 pulse rate,3 or renal function4-are also relevant to medicine, particularly in the design and interpretation of experimental work. Some rhythms have such low amplitude that they are not clinically important. Chronobiology is particularly important in the lung, however, since a circadian rhythm in airway resistance is easily demonstrated and, in asthma, is sometimes of such large amplitude that it is obvious to clinican and patient alike. The implications of this rhythm are threefold. It is relevant to the diagnosis of asthma, to the design and interpretation of trials of bronchodilator drugs, and, in severe cases, nocturnal asthma remains a challenge to pharmacology.