PROJECT SUMMARY PROJECT 1 (P1) Substance use disorders (SUD) are widely prevalent and pose devastating health, financial, and societal costs. The incidence of SU increases across adolescence, making this sensitive developmental period one of both heightened risk and heightened opportunity for prevention and intervention. However, to develop effective interventions, we need to identify novel and modifiable risk factors and mechanisms for SUD. Circadian rhythm and sleep disturbances have strong ties to SU risk, and their effects on intermediary markers of SU risk in adolescence—reward and inhibitory control systems—provides a plausible mechanistic substrate. The Center’s conceptual model posits that adolescent development is associated with enhanced reward function relative to cognitive control, phase delay in endogenous circadian rhythms, and lower homeostatic sleep drive. Environmental and social factors interact with these developmental processes, often resulting in late sleep timing, short sleep duration, and circadian misalignment—each of which is associated with increased substance use in teens and young adults. P1 will utilize the constant routine paradigm to rigorously characterize circadian rhythms and homeostatic sleep drive by controlling for masking influences of physical activity, posture, meals, and light levels. We will examine their individual and combined effects on measures of reward and cognitive control, and in combination with P2, on the development of substance use, to test the underlying mechanisms involved in the CARRS conceptual model. P1 will enroll 96 adolescents ages 13–15 (50% female) stratified by habitual sleep timing (early, intermediate, late, N=32 each). Participants will monitor sleep patterns at home with actigraphy and sleep diary, then complete fMRI measures of reward and cognitive control and a 60-hour lab session. The lab session includes two nights of polysomnographic (PSG) sleep studies, separated by 36 hours of sleep deprivation. After the first PSG, participants will follow a constant routine for the next 24 hours with wakeful bedrest; semi-recumbent posture; constant dim light; and hourly nutritional supplements. After 24 hours, participants will remain awake, but not confined to bed. Physiological circadian measures include salivary melatonin; core body temperature; and molecular rhythms from hair follicle cells (examined in P3). Physiological sleep homeostatic measures include waking EEG theta power. and delta sleep EEG response following 36 hours of wakefulness. Behavioral tests indexing cognitive control performance with and without reward modulation along with self-reports of mood and sleepiness will be collected every 2 hours. Finally, online surveys will index substance use every 6 months through study conclusion. P1 will draw directly on resources provided by the Center Cores. CARRS and P1 will innovatively advance understanding of distinct circadian and sleep homeostatic effects on reward-cognit...