COVID-19 has affected daily life in unprecedented ways. Using a longitudinal dataset linking biometric and survey data from several cohorts of young adults before and during the pandemic (N=685), we document large disruptions to physical activity, sleep, time use, and mental health. At the onset of the pandemic, average steps decline from 9,400 to 4,600 steps per day, sleep increases by about 25-30 minutes per night, time spent socializing declines by over half to less than 30 minutes, and screen time more than doubles to over 5 hours per day. The proportion of participants at risk of clinical depression increases to 65%, over twice the rate in the same population prior to the pandemic. Our analyses suggest that disruption to physical activity is a leading risk factor for depression during the pandemic. However, restoration of those habits–either naturally or through policy intervention–has limited impact on restoring mental well-being.