The sleep-circadian interface is a critical area of research in mental disorders, highlighting the reciprocal relationship between sleep, circadian rhythms, and mental health. Disruptions in sleep quality, continuity, and timing can exacerbate psychiatric symptoms, while treatments targeting sleep-circadian disturbances can alleviate psychopathology. Conversely, psychiatric symptoms can worsen sleep and disrupt circadian processes. Despite progress in understanding the mechanisms, a cohesive approach integrating sleep and circadian factors with psychiatric disorders is lacking. This review synthesizes recent evidence showing that sleep-circadian dysfunction is a transdiagnostic contributor to various psychiatric disorders, emphasizing biological mechanisms. It highlights the importance of early detection and intervention in adolescents and young adults, who are at high risk for mental disorders. The review aims to integrate sleep and circadian factors in mood, anxiety, and psychosis spectrum disorders, reframe existing knowledge, and identify research gaps. Sleep-circadian disturbances are common across psychiatric disorders and are linked to relapse and adverse outcomes. Laboratory studies show that sleep homeostasis and circadian processes contribute to diurnal mood variation. Intervention studies suggest that targeting sleep-circadian disturbances can improve psychiatric symptoms, though such interventions are not yet widely used. The review discusses the role of light in mental health, noting its effects on mood, alertness, and cognition. Circadian misalignment is associated with mood disorders and psychiatric symptoms, and its effects on sleep and circadian rhythms are significant. Neuroplasticity is also discussed, with sleep playing a key role in synaptic consolidation and memory. Insomnia is a prevalent sleep disorder linked to psychiatric disorders, with impaired emotional regulation and cognitive function. Noninvasive brain stimulation and light therapy are explored as potential interventions for psychiatric disorders. Chronopharmacology suggests that medication timing can optimize efficacy and minimize adverse effects. Timed meals and exercise are also considered for their impact on mental health. Future research should focus on integrating sleep and circadian factors, developing common standards for biomarker collection, and exploring the interaction between sleep-circadian disruption and environmental factors. The review emphasizes the need for a coordinated research agenda to advance understanding and treatment of mental disorders.The sleep-circadian interface is a critical area of research in mental disorders, highlighting the reciprocal relationship between sleep, circadian rhythms, and mental health. Disruptions in sleep quality, continuity, and timing can exacerbate psychiatric symptoms, while treatments targeting sleep-circadian disturbances can alleviate psychopathology. Conversely, psychiatric symptoms can worsen sleep and disrupt circadian processes. Despite progress in understanding the mechanisms, a cohesive approach integrating sleep and circadian factors with psychiatric disorders is lacking. This review synthesizes recent evidence showing that sleep-circadian dysfunction is a transdiagnostic contributor to various psychiatric disorders, emphasizing biological mechanisms. It highlights the importance of early detection and intervention in adolescents and young adults, who are at high risk for mental disorders. The review aims to integrate sleep and circadian factors in mood, anxiety, and psychosis spectrum disorders, reframe existing knowledge, and identify research gaps. Sleep-circadian disturbances are common across psychiatric disorders and are linked to relapse and adverse outcomes. Laboratory studies show that sleep homeostasis and circadian processes contribute to diurnal mood variation. Intervention studies suggest that targeting sleep-circadian disturbances can improve psychiatric symptoms, though such interventions are not yet widely used. The review discusses the role of light in mental health, noting its effects on mood, alertness, and cognition. Circadian misalignment is associated with mood disorders and psychiatric symptoms, and its effects on sleep and circadian rhythms are significant. Neuroplasticity is also discussed, with sleep playing a key role in synaptic consolidation and memory. Insomnia is a prevalent sleep disorder linked to psychiatric disorders, with impaired emotional regulation and cognitive function. Noninvasive brain stimulation and light therapy are explored as potential interventions for psychiatric disorders. Chronopharmacology suggests that medication timing can optimize efficacy and minimize adverse effects. Timed meals and exercise are also considered for their impact on mental health. Future research should focus on integrating sleep and circadian factors, developing common standards for biomarker collection, and exploring the interaction between sleep-circadian disruption and environmental factors. The review emphasizes the need for a coordinated research agenda to advance understanding and treatment of mental disorders.