Characterizing ActiGraph's Idle Sleep Mode in Free-Living Assessments of Physical Behavior

ActiGraph's idle sleep mode (ISM) is an optional battery- and memory-conserving feature believed to engage during periods of nonwear, inactivity, and sleep, but it has not been well studied in free-living environments. Thus, we investigated ISM during a 7-day assessment in a nationally representative sample of 13,649 participants (6-80 years) in the United States and found it engaged 43.6% +/- 0.2% (mean +/- SE) of the 24 hr per day. ISM engagement was highest (78.4% +/- 0.2%) during early morning (00:00-05:59) and lowest (20.4% +/- 0.3%) during afternoon (12:00-17:59), corresponding to quadrants of lowest and highest of movement, respectively. ISM engagement was also inversely correlated with daily activity across all participants (R = -.72, p < .001). When restricted to participants averaging >= 21 hr per day of wear (N = 10,482), ISM still engaged 39.5% +/- 0.2% of the day and inversely correlated to daily activity (R = -.58, p < .001). These results suggest ISM engages in activity level-dependent temporal patterns. Additional research is needed to better inform analyses and interpretation of ISM-enabled data including whether it is appropriate to process them with existing methods that were developed and validated using data without ISM enabled. This issue may be particularly relevant for methods used to detect and score sleep, as ISM engaged during a substantial portion of the typical overnight sleep period in the 8-hr window between >= 22:00 and <06:00 (74.0% +/- 12.6%, mean +/- SD).