Nocturnal motor breathing and cardiac activity were recorded by using the static charge-sensitive bed, and sleep habits were studied by questionnaire in 24 pairs of monozygotic twins discordant for their body mass index; the mean intrapair difference between co-twins was 6.7 kg/m2. Intrapair differences in sleep characteristics between co-twins were related to intrapair differences in physiological and anthropometric characteristics. Two questions were tested. First, do monozygotic twins discordant for body mass index differ in sleep behavior? Second, if they do, are differences in sleep associated with nongenetic differences in the body weight and metabolism or with other environmental factors? Obese twins had higher nocturnal motor activity levels, less quiet sleep, and more habitual snoring than did their nonobese co-twins. Differences in sleep were associated with obesity-related factors. However, habitual snoring did not explain other intrapair differences in sleep. It was concluded that relatively moderate obesity is associated with disruption of physiological structure of sleep as measured by the static charge-sensitive bed method and that this disruption seems not to be associated with snoring or breathing disturbances.