In my reading and my own experience (using a Galaxy watch), this is almost certain, not absolutely certain, to be attributable to one of two conditions:
a. Sleep apnea - where your heart has to race to keep you alive because your serum O2 levels drop when you cannot get air past your pharyngeal tissues and nasal tissues, or past your upper trachea; or
b. Dreams....REM sleep stage. When we dream, all sorts of unknown/unseen things happen, from sexual stimulation/arousal, to thrashing, to calling out, to swinging wildly, to vocalizing, and feeling terror or joy. If the dream is quite vivid, and would otherwise cause waking arousal of any kind, stimulation, then the body treats it the same way...increased respiration and heart rate, adrenalin production, increase in serum cortisol, etc.