It Shows I Had Night Steps, Though I Know I Didn't

Sleep Pulse reads "Steps" directly from Health, which are recorded with Apple's algorithm. Apple, as of 2017, doesn't "expect" anyone to sleep with Apple Watch. Therefore, in testing, it seems that if exaggerated movements occur with the arm your Apple Watch is on, these may register as "Steps" while you sleep. 

To limit false positives, Sleep Pulse only looks for Night Steps between the range of twenty minutes after you fell asleep and twenty minutes before you wake up.