Twin cross-trait correlations of sleep problems and gastroesophageal reflux disease.

aPolychoric correlation.bAsymptotic error of the polychoric correlations.cThe insomnia index was constructed by combining the three different sleep problem questions and 0 points were given for “seldom”, 1 point for “sometimes” and 2 points for “often”. The highest scores, 4–6 points or if the participant had answered “often”, were then classified as “often having sleep problems”, 1–3 points as “sometimes having sleep problems” and 0 as ”seldom having sleep problems”.