In obese males obstructive sleep apnoea (OSA) is associated with inflammation and insulin resistance; however, findings are confounded by adipose tissue, a hormone- and cytokine-secreting organ. Our goal was to examine whether in a relatively nonobese population, OSA is associated with sleepiness and inflammation/insulin resistance, and to assess the effects of a 2-month placebo-controlled continuous positive airway pressure (CPAP) use. 77 subjects, 38 middle-aged males and post-menopausal females with OSA and 39 male and female controls, were studied in the sleep laboratory for 4 nights. Measures of sleepiness (objective and subjective), performance, serial 24-h blood samples for interleukin (IL)-6, tumour necrosis factor receptor (TNFR)-1, leptin and adiponectin, and single samples for high-sensitivity C-reactive protein (hsCRP), fasting glucose and insulin levels were obtained. Apnoeic males were significantly sleepier and had significantly higher hsCRP, IL-6, leptin and insulin resistance than controls. Apnoeic females had significantly higher hsCRP; however, objective sleepiness, IL-6, TNFR-1, insulin resistance (Homeostatic Model Assessment index), leptin and adiponectin were similar to controls. CPAP improved subjective sleepiness, but no changes were observed in any of the biomarkers. In conclusion, OSA is associated with sleepiness, inflammation and insulin resistance, even in nonobese males, and this association is stronger in males than in females. Short-term CPAP does not improve the inflammatory/metabolic aberrations in OSA.