Insomnia in Vietnam era veteran twins: influence of genes and combat experience.

Genetic and environmental influences on insomnia were studied in 2,825 pairs of Vietnam era veteran male twins. The self-reported sleep problems studied included trouble falling asleep, trouble staying asleep, waking often, waking tired and a composite sleep scale. Twin correlations for each of the sleep problems were larger in monozygotic than in dizygotic pairs, with heritability estimates ranging from 0.21 to 0.42. There was no effect of common familial environment. Phenotypic correlations for combat experience and sleep problems were small, ranging from 0.00 to 0.09, with no differences seen in monozygotic and dizygotic twins. When the effects of genes and combat exposure were evaluated simultaneously, there was a significant genetic contribution to all sleep measures, but combat exposure was significantly associated only with overall sleep quality, waking often and having trouble staying asleep.