The twin design is most powerful for estimating the genetic and/or environmental overlap between physiological measurements such as sleep-recorded disordered breathing, obesity, and craniofacial morphology. The investigators chose to focus on an elderly male twin sample for which a wealth of data relevant to this study had been previously collected. Twin pair concordance or discordance for monitored sleep-disordered breathing allowed the full characterization of genetic/familial and individual environmental factors associated with the expression and severity of this condition.