Purpose: Insomnia affects up to one in four adolescents and has been shown to have a negative impact on their mental and physical health. This study aimed to investigate the association between insomnia, academic performance, self-reported health, physical activity, school start time, and substance use among adolescents. Methods: A survey with a cross-sectional design was completed by adolescents (15-17 years old; n = 1504) in southern Sweden. The Minimal Insomnia Symptoms Scale (MISS) was used to operationalize insomnia. A multiple logistic regression analysis was used to analyze the relationship between insomnia and self-reported health, failed school courses, substance use, school start time, family financial situation, screen time, and gender. Results: Insomnia (MISS >= 6) was associated with poor self-reported health (OR: 4.35), failed school courses (OR: 1.47), and use of alcohol and/or cigarettes (OR: 1.43). When the combined effect of self-reported health and physical activity were investigated, a combination of low physical activity (<= 1 time/week) and poor self-reported health was strongly associated with insomnia (OR: 18.87). Conclusions: Insomnia was associated with other problems that in themselves are risk factors for poor health. This highlights the need for a holistic health-promoting approach to prevent insomnia, such as efforts to promote physical activity, school success, and the reduction of alcohol/cigarette use.