The literature search was conducted using Google Scholar and PubMed with relevant search terms including: Circadian rhythms, Substance abuse, Substance use disorder, Clock genes, Dopaminergic pathways, Chronotype, Alcohol use disorder, Opioids, Effect of circadian rhythm disruption on substance use, Effect of artificial light on circadian system, Substance use disorder in adolescents, and follow-up searches as warranted. This evidence concerns the gene CLOCK and substance abuse.