Both sleep duration and efficiency are essential for health outcomes. However, few studies have considered the effects of both sleep duration and efficiency on predicting the risks of mortality. This study investigated the independent and joint associations of accelerometer-measured sleep duration and efficiency with all-cause and cause-specific mortality.The UK Biobank is a cohort study of over 500 000 individuals recruited between 2006 and 2010. This study included participants wearing wrist accelerometers for 7 consecutive days between February 2013 and December 2015. Mortality was ascertained by the national death registries.90 398 participants (age, 62.4 [7.8] years, 43.5% male) were included, 2 685 deaths were reported within a median follow-up duration of 6.4 years. Both accelerometer-measured short (adjusted hazard ratios [HR], 1.27; 95%CI, 1.11 to 1.45) and long sleep duration (adjusted HR, 1.16; 95%CI, 1.06 to 1.28) was positively associated with the risks of all-cause mortality. Lower sleep efficiency was associated with an increased risk of all-cause and cause-specific mortality. Significant interaction existed between accelerometer-measured sleep duration and efficiency for the risk of all-cause mortality (Pinteraction = 0.001), participants with long sleep duration and lower sleep efficiency had a double mortality risk compared with those with higher sleep efficiency and normal sleep duration (adjusted HR = 2.11; 95%CI, 1.44 to 3.09).Accelerometer-measured short/long sleep duration and lower sleep efficiency were associated with increased risks of mortality. Sleep efficiency modified the effects of long sleep duration on survival.© The Author(s) 2023. Published by Oxford University Press on behalf of The Gerontological Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.