In addition, we compared all subjects who reported sleeping or were noted to have fallen asleep by the experimenter (n = 252) to the remaining subjects with respect to different measures of EEG power in a series of univariate analyses using lme4 in R to conduct a linear mixed model (LMM) with a random intercept at the family level to accommodate the dependency of EEG measures within families. Age, gender, age cohort, and recording system were included as covariates (see below). Eleven of the 12 LMMs yielded significant effects, with a clear trend: subjects observed to fall asleep had greater low frequency power (theta and delta, t-statistics ranging from 2.64 to 9.28 across electrodes and the two bands) and less high frequency power (alpha and beta, t-statistics ranging from −0.90 to −5.08 across electrodes and bands). Only beta power at 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
