--- - |2+ Background Insomnia has been associated in cross‐sectional studies with increased beta (15–35 Hz) electroencephalogram (EEG) power during nonrapid eye movement (NREM) sleep, an index of cortical hyperarousal. However, it is unknown whether this cortical hyperarousal is present before individuals with insomnia develop the disorder. To fill this gap, we examined the association of childhood sleep high‐frequency EEG activity with incident insomnia symptoms (i.e., absence of insomnia symptoms in childhood but presence in adolescence). Methods We studied a case–control subsample of 45 children (6–11 years) from the Penn State Child Cohort, a population‐based random sample of 421 children, who were followed up after 8 years as adolescents (13–20 years). We examined low‐beta (15–25 Hz) and high‐beta (25–35 Hz) relative power at central EEG derivations during NREM sleep and, in secondary analyses, during sleep onset latency, sleep onset, and REM sleep. Incident insomnia symptoms were defined as the absence of parent‐reported difficulty falling and/or staying asleep during childhood and a self‐report of these insomnia symptoms during adolescence. Results Childhood high‐beta power during NREM sleep was significantly increased in children who developed insomnia symptoms in adolescence (n = 25) as compared to normal sleeping controls (n = 20; p = .03). Multivariable‐adjusted logistic regression models showed that increased childhood high‐beta EEG power during NREM sleep was associated with a threefold increased odds (95% CI = 1.12–7.98) of incident insomnia symptoms in adolescence. No other significant relationships were observed for other sleep/wake states or EEG frequency bands. Conclusions Increased childhood high‐frequency EEG power during NREM sleep is associated with incident insomnia symptoms in adolescence. This study indicates that cortical hyperarousal during sleep may be a premorbid neurophysiological sign of insomnia, which may mediate the increased risk of psychiatric disorders associated with insomnia. - Journal of Child Psychology and Psychiatry, EarlyView.