This study compares the accuracy, sensitivity, and specificity of wrist actigraphy with polysomnography (PSG) in measuring sleep and wakefulness. The research involved 77 participants, including young and older adults, healthy individuals, and those with chronic primary insomnia, as well as 23 night workers. Actigraphy and PSG data were collected simultaneously during sleep laboratory admissions, with all studies involving an 8.5-hour time in bed. The analysis focused on 30-second epochs to assess sensitivity (actigraphy = sleep when PSG = sleep), specificity (actigraphy = wake when PSG = wake), and accuracy (total proportion correct). The study found high sensitivity (0.965) and accuracy (0.863) but low specificity (0.329). Increasing age slightly reduced specificity, and actigraphy overestimated wakefulness after sleep onset (WASO) compared to PSG. The study concludes that actigraphy is a useful and valid tool for estimating total sleep time and WASO in clinical and population studies, despite some limitations in specificity.This study compares the accuracy, sensitivity, and specificity of wrist actigraphy with polysomnography (PSG) in measuring sleep and wakefulness. The research involved 77 participants, including young and older adults, healthy individuals, and those with chronic primary insomnia, as well as 23 night workers. Actigraphy and PSG data were collected simultaneously during sleep laboratory admissions, with all studies involving an 8.5-hour time in bed. The analysis focused on 30-second epochs to assess sensitivity (actigraphy = sleep when PSG = sleep), specificity (actigraphy = wake when PSG = wake), and accuracy (total proportion correct). The study found high sensitivity (0.965) and accuracy (0.863) but low specificity (0.329). Increasing age slightly reduced specificity, and actigraphy overestimated wakefulness after sleep onset (WASO) compared to PSG. The study concludes that actigraphy is a useful and valid tool for estimating total sleep time and WASO in clinical and population studies, despite some limitations in specificity.