ABSTRACT Background Sleep disturbance is common in adolescents and young adults (AYAs), impacted by stress and academic/scheduling demands that conflict with biological phase delay. COVID-19 lockdowns allowed us to study sleep in AYAs when there are lessened scheduling demands. Additionally, we could test whether a sleep self-management app was helpful during lockdowns. Method AYAs (ages 15–24 years) before (Pre-Lockdown; n = 65) and during lockdowns in Canada (Lockdown; n = 40) completed sleep diaries on the app; set goals based on generated feedback; and completed more sleep diaries pursuing whatever post-feedback goals they set. Results The Lockdown group reported later and less variable rise times (RT) and spent more time in bed (TIB), both awake and asleep. Pre-Lockdown set a goal to reduce RT variability whereas Lockdown set a goal to decrease TIB, and AYAs made behaviour changes to meet their goals. For both groups, sleep onset, duration of awakenings, sleep duration and efficiency, and insomnia severity significantly improved at endpoint. Conclusions AYAs slept differently during lockdowns, perhaps due to decreased scheduling. The pandemic revealed the need for accessible strategies to improve sleep health. Findings support the feasibility of using evidence-based apps, and that AYAs can effectively use self-management tools across variable global and social contexts to improve their sleep.