Sleep Pulse uses a complex tested algorithm that looks across your sleep for motion and heart rate patterns. To calculate your sleep analysis, your sleep is parted into sections: one of very limited movement or "stillness" and another of all other movement. From the sensitivity you select for your sleep analysis, a threshold is set on the motion data, storing it as light or restless.
Your "Slept" duration includes the deep + light sleep durations. Your heart rate, though limited in the background to measurements every 5 - 10 minutes, helps set the REM Estimate and Fell Asleep Heart Rate.