This study compares wrist actigraphy to polysomnography (PSG) in measuring sleep. Actigraphy, which uses a wrist-worn device to track movement, was compared to PSG, the gold standard for sleep measurement. The study involved 77 participants, including young and older adults, healthy individuals, and those with chronic primary insomnia. Data were collected during sleep laboratory visits, with both actigraphy and PSG simultaneously recorded. The study evaluated accuracy, sensitivity, and specificity of actigraphy in detecting sleep and wakefulness, as well as wake after sleep onset (WASO). Overall, actigraphy showed high accuracy (86.3%) and sensitivity (96.5%), but lower specificity (32.9%). These metrics were only slightly affected by factors such as age, gender, and sleep timing. Actigraphy tended to overestimate sleep and underestimate wakefulness, particularly when WASO was high. The study found that actigraphy was more accurate in participants with higher sleep efficiency and less accurate in those with lower sleep efficiency. Actigraphy was also less accurate in older women with insomnia. The study validated the use of actigraphy for estimating total sleep time and WASO in clinical and population studies, despite its limitations in specificity. Actigraphy was found to be a useful and valid tool for measuring sleep in field and workplace studies, but with some limitations in specificity. The study also found that actigraphy tends to underestimate WASO when it is greater than 30 minutes. The results suggest that actigraphy can be a valuable tool for measuring sleep in various research and clinical settings, particularly for longitudinal studies and for individuals with insomnia or circadian rhythm disorders. However, the study acknowledges the limitations of actigraphy, including its lower specificity and potential underestimation of WASO. The study also highlights the need for further research to determine the effectiveness of actigraphy in measuring sleep over a 24-hour period and its potential use in assessing daytime napping.This study compares wrist actigraphy to polysomnography (PSG) in measuring sleep. Actigraphy, which uses a wrist-worn device to track movement, was compared to PSG, the gold standard for sleep measurement. The study involved 77 participants, including young and older adults, healthy individuals, and those with chronic primary insomnia. Data were collected during sleep laboratory visits, with both actigraphy and PSG simultaneously recorded. The study evaluated accuracy, sensitivity, and specificity of actigraphy in detecting sleep and wakefulness, as well as wake after sleep onset (WASO). Overall, actigraphy showed high accuracy (86.3%) and sensitivity (96.5%), but lower specificity (32.9%). These metrics were only slightly affected by factors such as age, gender, and sleep timing. Actigraphy tended to overestimate sleep and underestimate wakefulness, particularly when WASO was high. The study found that actigraphy was more accurate in participants with higher sleep efficiency and less accurate in those with lower sleep efficiency. Actigraphy was also less accurate in older women with insomnia. The study validated the use of actigraphy for estimating total sleep time and WASO in clinical and population studies, despite its limitations in specificity. Actigraphy was found to be a useful and valid tool for measuring sleep in field and workplace studies, but with some limitations in specificity. The study also found that actigraphy tends to underestimate WASO when it is greater than 30 minutes. The results suggest that actigraphy can be a valuable tool for measuring sleep in various research and clinical settings, particularly for longitudinal studies and for individuals with insomnia or circadian rhythm disorders. However, the study acknowledges the limitations of actigraphy, including its lower specificity and potential underestimation of WASO. The study also highlights the need for further research to determine the effectiveness of actigraphy in measuring sleep over a 24-hour period and its potential use in assessing daytime napping.