This study investigated the relationship between the level of daytime activities and nocturnal sleep in the elderly using the wrist actigraph and ambulatory polysomnograph (PSG) system. Ten male and ten female subjects (mean age ± SE=73.2 ± 1.62 years; range 65-81 years) participated in this study. The level of daytime activity was positively correlated with the sleep efficiency of the subsequent night. Based on the amount of daytime activity level on the PSG recording night, 6 participants were selected as the high activity group, and other 6 participants selected as the low activity group. The duration of slow wave sleep (SWS) was significantly longer for high activity group than those for low activity group. Moreover, the duration of wake after sleep onset was longer for low activity group than those for high activity group. The results imply that increment the time of physical activities may improve the quality of nocturnal sleep for the elderly subjects.