Insomnia among adolescents is a prevalent public health concern and is closely linked to suicidal tendencies, health risk behaviors, and other adverse health outcomes. This study builds on existing literature by exploring the multifaceted associations between insomnia and socio-behavioral factors, which are currently underexplored in the Nepalese context. It assesses the prevalence of insomnia and its association with socio-behavioral factors and internet addiction among adolescents in Bagmati Province, Nepal, to inform targeted public health interventions. From July to September 2022, a school-based descriptive cross-sectional study was conducted among grade 9 and 10 students (aged 13-19) using a self-administered semi-structured questionnaire. A questionnaire included the 7-item Insomnia Severity Index (ISI) for insomnia assessment and the 20-item Young's Internet Addiction Test for evaluating internet addiction. Binary logistic regression analysis was utilized to identify factors associated with insomnia. The study identified a significant prevalence of insomnia at 24.2%. Key socio-behavioral determinants included religion [AOR 3.58; 95% CI 1.56-8.23, AOR 3.36; 95% CI 1.27-8.89], experience of a break up [AOR 1.67; 95% CI 1.10-2.55] absence of close friendships [AOR 2.62; 95% CI 1.32-5.19], exposure to bullying [AOR 1.74, 95% CI 1.12-2.70], and internet addiction [AOR 2.74; CI 1.83-4.11]. These findings highlight the complex interplay of individual and environmental factors influencing insomnia. The significant prevalence of insomnia among school-going adolescents in Bagmati Province underscores the necessity for enhancing the role of schools in health counselling that considers behavioural, social, and demographic factors. Addressing internet addiction, fostering healthy social connections, and acknowledging the impact of demographic factors like religion could enhance intervention strategies.