iSPHYNCS: Enabling long-term actigraphy with vital signals in a comparison between Fitbit Inspire 2/HR and MotionWatch 8 for non-parametric circadian rhythm analysis

Actigraphy is a tool to study an individual’s rest-activity rhythm, but its use is limited secondary to both cost and its availability to only specialized medical centers. Consumer-grade activity trackers are widely used by the general population. However, it remains to be determined if these devices provide equivalent information compared to traditional actigraphy and if the additional measurements of vital signals may further facilitate the identification of specific sleep disorders. We propose a new approach to perform long-term actigraphy with consumer-grade activity trackers that provide a broader array of physiological vital signals. We recorded one week of simultaneous actigraphy (MotionWatch 8) and activity tracking (Fitbit Inspire HR/2) in a study population (n $=$ 40) that included individuals with central disorders of hypersomnolence and healthy controls. We leverage information from the individuals’ heart rate, step count, and calorie consumption measured by Fitbit to calculate non-parametric circadian rhythm analysis. Performing Bland-Altman analyses to assess the agreement between the clinical actigraphy and our proposed methods, we show that calorie consumption of Fitbit offers an acceptable alternative to the actigraphy device, superior to step count. Our results suggest that consumer-grade activity trackers have the potential to expand current medical device-based actigraphy by enabling additional signal detection and longer recording times at a lower cost.