actigraph

noun

ac·​ti·​graph ˈak-tə-ˌgraf 

: a small device usually worn on the wrist that records the activity level of the body by sensing physical movement and that is used especially to measure the amount and quality of sleep

Researchers measured the sleep time of 2,978 women by equipping them with an actigraph, a watch-size device worn on the wrist that measures periods of activity and inactivity.

—Nicholas Bakalar, The New York Times

Actigraph data accurately identify periods of sleep and wakefulness and yield estimates of movement, the researchers say.

—Bruce Bower, Science News

Note: An actigraph consists of an accelerometer and a memory unit and is used to collect data typically for a period of several days to one or two weeks.