Nocturnal symptoms are common in asthma, even when patients are regularly seen at an outpatient clinic. Inflammation is generally accepted as a general feature of asthma and the severity of this basic inflammatory process can be increased by exogenous triggers such as exposure to allergens and nonallergic stimuli. Superimposed endogenous circadian rhythms may play a more important and intricate role in the circadian modulation of the inflammatory process by changing the number of cells, their release of mediators and/or the susceptibility of airway smooth muscle and vasculature. For instance, an increase in vagal tone may induce nocturnal bronchoconstriction which is further enhanced by falling catecholamine levels. Together, the reduced nocturnal catecholamine levels and the diminished bronchodilating capacity of the NANC system and the low cortisol levels oppose possible protection against inflammatory processes leading to nocturnal airflow obstruction.