High heritability of adolescent sleep-wake behavior on free, but not school days: a long-term twin study.

Adolescence development is characterized by significant changes in sleep biology. Despite an overall decline in sleep duration and a delay in bedtime, significant interindividual variation in sleep has been reported. The aim of the current study was to examine genetic and environmental influences on sleep in adolescence using long-term (6 month) actigraphy measurements, differentiating between school and free days. Sixteen monozygotic (n = 32) and 10 dizygotic (n = 20) twin pairs (mean age 12.8 ± 1.0 years; 25 females) participated in the study. Structural equation modeling was used to compute genetic, shared environmental and unique environmental contributors to sleep behavior. We found significantly more genetic influence on sleep timing (sleep midpoint; school: 14%, free: 90%) and duration (school: 15%; free: 68%) on free compared with school days. On the other hand, the genetic influence on measures of sleep quality (sleep efficiency and sleep onset latency) was high (>60%) and less dependent on the day of measurement. Only wake after sleep onset (WASO) exhibited a strong shared environmental influence (> 52%) on both school and free days, suggesting that behavioral/environmental interventions may help reduce WASO. In addition, self-reported chronotype was also highly genetically influenced (75%). Disrupted, ill-timed, and insufficient sleep in adolescence is associated with poor mental and physical health outcomes. Our findings of a strong genetic contribution to sleep in adolescence suggest that sleep may mark a genetic vulnerability to poor outcomes.