How did you sleep? Find out with the Index Sleep Monitor. Comfortable, lightweight and worn on the upper arm, this sleep armband can capture sleep-tracking and recovery metrics including sleep score, sleep stages and HRV status so you can better understand how your body recovers ...How did you sleep? Find out with the Index Sleep Monitor. Comfortable, lightweight and worn on the upper arm, this sleep armband can capture sleep-tracking and recovery metrics including sleep score, sleep stages and HRV status so you can better understand how your body recovers each night (data presented is intended to be a close estimation of metrics tracked).