Sleep Pulse 3 on Apple Watch uses your motion and heart rate to analyze your sleep. As of watchOS 4, to retrieve this data, a motion "session" must be started and stopped by a user while the app is active. Meaning to accurately track your sleep — auto-tracking, for now, is not possible due to software limitations.
Any app that claims to accurately "auto-track your sleep" is misleading and should be avoided unless the main goal of said "sleep tracker" is solely to track when you go to bed and when you awake. Said apps only use your Heart Rate (only available every 5-10 min in the background on watchOS) to try and estimate your sleep based on a falling and raising rate when you sleep and wake in the morning. Any data of your sleep in between those two points will be almost useless without a motion analysis also.
All apps that claim to "auto-track" your sleep have been tested by the Sleep Pulse team, and motion is not used in the analysis. (Try turning off the motion permission for these said "sleep trackers" and you will notice they still somehow track your sleep).