Patients with nocturnal asthma demonstrate circadian variations in airway inflammation. We hypothesized that melatonin, a circadian rhythm regulator, modulates circadian inflammatory variations in asthma. The effect of melatonin stimulation on peripheral blood mononuclear cell cytokine production was evaluated in normal controls, nocturnal asthmatics and non-nocturnal asthmatics at 4 p.m. and 4 a.m. Melatonin was pro-inflammatory, causing significantly increased production of IL-1, IL-6 and TNF- at 4 p.m. and 4 a.m. in all subject groups (range 12.8±3.3% to 131.72±16.4%, p0.0003). The observed increases in cytokine production did not change between 4 p.m. and 4 a.m. in controls or nocturnal asthmatics (p>0.05, both cases). At 4 p.m., the nocturnal asthmatic cytokine response to melatonin was greater than controls or non-nocturnal asthmatics and did not change significantly at 4 a.m. At 4 p.m., non-nocturnal asthmatic cytokine response was less than nocturnal asthma and rose significantly at 4 a.m. (p=0.0001, all comparisons). Melatonin is proinflammatory in both asthmatic and healthy subjects. Nocturnal asthmatics demonstrate the largest daytime cytokine response and cannot be further stimulated at 4 a.m., suggesting chronic overstimulation in vivo. These results suggest differential immunomodulatory effects of melatonin based on asthma clinical phenotype and may indicate an adverse effect of exogenous melatonin in asthma.