Sleep and Circadian Rhythms, Actigraphy
This variable is only available in MAP.
kRA is a measure of transitioning from a state of rest to a state of activity and serves as a proxy for measuring sleep fragementation. Transition probabilities are estimated for both activity-to-rest and rest-to-activity states; this is the rest-to-activity variable. kRA is measured using a portable actigraph: AX3 (2020-present), GENEActiv (2018-2020), Actical (2005-2018). See the Actigraphy category description for detailed information on each device.
Participants are asked to wear the actigraph 24 hours per day for up to 10 days. The device records activity counts for 1-second samples and sums the counts into 15-second intervals called epochs. Each epoch is classified as resting (0 counts per epoch) or active (greater than 0 counts per epoch).
Runs of rest begin with at least one epoch of rest and end at the epoch before the first epoch of activity. That is, each run of rest begins with an activity-to-rest transition and ends with a rest-to-activity transition. For each individual record, the total numbers and durations of runs of rest are used to calculate the probability of transitioning from sustained rest to activity. kRA is then obtained by calculating the weighted average value of the probabilities. See reference for details on the calculation of kRA.
The higher the kRA, the more poorly sustained are runs of rest, and the lower the kRA, the more consolidated they are.
Note: Periods during which the actigraph was not worn is removed from analysis by flagging periods with greater than 4 consecutive hours of no movement and excluding the entire 24 hour period surrounding the flagged period.