Begin tracking only after no motion is felt for a set time.
I fall asleep at different lengths of time. Sometimes 20 minutes sometimes 3+ hours. If there was an option to only start tracking after no movement is felt for a specified time then the information would be more accurate. This option should let the user pick the amount of time delay between movements before tracking starts.
I suggested a similar feature to vibrate a minute or two before a initial default pause runs out, would work best with a wearable so then you can extend the pause another 15-20 mins.
Your first sleep phase isn't necessarily motionless. So it could not kick in until deeper sleep phases.
I think this would result in less accurate sleep tracking.