Abstract Objective To prospectively document changes in adolescents’ sleep before versus during the COVID-19 pandemic, and to examine their impact on adolescents’ perceived stress. Methods Sixty-two typically developing adolescents participated in the study before (Time 1: January 15 to March 13, 2020) and during (Time 2: May 15 to June 30, 2020) the COVID-19 pandemic in Canada. At Time 1, each participant’s sleep pattern was assessed in the home environment using actigraphy and sleep logs for seven consecutive nights. Adolescents completed a battery of questionnaires in which they reported on their sleep schedule, duration, and quality, as well as their activities at bedtime, their daytime sleepiness, and their social/emotional behavior. The participants’ parents provided demographic information. At Time 2, each participant completed a sleep log, the same battery of questionnaires regarding sleep, and the Perceived Stress Scale. Results (1) Adolescents’ reported sleep was of longer duration and on a delayed schedule during the COVID-19 pandemic compared to pre-pandemic. (2) A larger proportion of adolescents reported meeting or exceeding the recommended amount of sleep during the COVID-19 pandemic compared to pre-pandemic sleep. (3) “Social jet lag” disappeared during the COVID-19 pandemic. (4) A shorter reported sleep duration and higher level of arousal at bedtime at Time 1 were significant predictors of adolescents’ perceived stress at Time 2—during the COVID-19 pandemic. (5) A higher levels of arousal at bedtime and lower reported sleep quality at Time 2 were concurrently associated with higher levels of perceived stress among adolescents, even when we controlled for the levels of pre-pandemic emotional or behavioral issues, sleep duration, or sleep quality. Conclusion Sleep duration and cognitive-emotional arousal, which are both modifiable behaviors, were associated with adolescents’ perceived stress during the COVID-19 pandemic. These behaviors could be useful targets for preventive interventions aiming to reduce adolescents’ stress in the face of stressogenic situations, such as the COVID-19 pandemic.