Cz did not show a significant effect, although the direction of effect was the same as the direction in the occipital-parietal locations. In addition, peak alpha frequency at the occipital sites (described below) was slower among those reported to sleep (t-statistics of −2.62 and −2.99). As a result, these subjects were excluded. The final sample consisted of 4,026 individuals, 2,383 adolescents (1,153 males) and 1,643 adults (903 males) from 1,613 families. The majority of families were MZ twin families (1038, or 64%). The mean age was 17.7 (range, 16.6–20.0) for adolescent participants and 45.1 (range, 29.6–65.3) for the parents.
