Single question about total lying time for assessing physical inactivity in community-dwelling older adults: a study of reliability and discriminant validity from sleeping time

[Purpose] To investigate reliability and discriminant validity of a single question about total lying time for assessing physical inactivity in community-dwelling older people. [Participants and Methods] The participants were 54 healthy older individuals (mean age, 72.5 years), who were asked to recall retrospectively their mean total lying and sleep times per day in the previous week (7 days). The total lying and sleep times per day in the forthcoming week (7 days) were also investigated prospectively after confirming the mean total lying and sleep times per day in the previous week, and their mean values per day were calculated. [Results] Intraclass reliability of total lying and sleep times per day in the forthcoming week were acceptable [ICC (1, 1) for total lying time=0.835, ICC (1, 1) for sleep time=0.707]. No significant difference in average total lying time between the previous (8.4 ± 2.0 hours/day) and forthcoming (8.7 ± 1.7 hours/day) weeks was seen. In the forthcoming week, average total lying time was significantly higher than average sleep time (7.1 ± 1.3 hours/day). There was low significant correlation between total lying time and sleep time. [Conclusion] Total lying time can be measured with acceptable reliability and discriminant validity, and is a different outcome than sleep time in community-dwelling older adults.