The investigators recruited a subsample of pairs from the NAS-NRC World War II Twin Registry in which at least one of the twin brothers reported sleep apnea symptoms and a subsample of control pairs in which both twins reported no symptoms. They monitoring the subgroup of twin pairs with overnight sleep recording. They also collected anthropometric measurements of weight, height, neck circumference, and craniofacial morphology and blood samples for determination of zygosity and DNA extraction for future molecular studies.