A new study reports that a habitual short sleep duration of less than five hours and obstructive sleep apnea are independently associated with abdominal visceral fat in Korean adults.   Published in the May issue of Sleep, the study found that the association is particularly strong in short sleepers with OSA, who are four times more likely to have visceral obesity than those without OSA who slept seven hours or more.  Because of its cross-sectional design, the study was unable to characterize the causal relationship between sleep disorders and obesity measures.  However, the authors concluded that the study suggests a possible mechanism of increased metabolic risks in those with short sleep duration and OSA.