Our understanding of obstructive sleep apnoea (OSA) has advanced much beyond that of the condition being a noisy breathing abnormality during sleep which classically presents with daytime sleepiness, to a condition with numerous potentially significant downstream sequelae and multisystem manifestations. The relationship between OSA and metabolic dysfunction is a moving target. Cross sectional studies have provided suggestive evidence for an independent relationship between OSA and adverse glucose metabolism, while supportive longitudinal data and interventional data from randomised controlled trials are still lacking.1 Recently, data from the Sleep Heart Health Study again confirmed the independent association between sleep disordered breathing and impaired glucose metabolism in normal weight and overweight/obese individuals.2 In this issue of Thorax, the study of Barceló and colleagues3 suggested that the presence/absence of daytime sleepiness determined the presence/absence of increased insulin resistance in OSA, and that continuous positive airway pressure (CPAP) treatment decreased insulin resistance in sleepy subjects but had no effect in non-sleepy subjects (see page 946). Their findings showed a pattern analogous to the previously reported phenomenon regarding OSA and elevated blood pressure, whereby the severity of daytime sleepiness in OSA predicted a blood pressure lowering response to CPAP treatment.4 5

Excessive daytime sleepiness has always been considered as one of the cardinal symptoms of OSA, and it is usually improved by effective treatment of OSA. However, it has also been well recognised that excessive daytime sleepiness is not universally present in OSA …