How about because I live a healthy life with plenty of exercise along with meditating on a routine basis. My heart rate reads at 58bpm but is working on getting a more efficient rate.
REM rebound is not normal or associated with exercise or meditation. I'm not saying you are unhealthy in a general sense, I am simply saying your sleep deprivation is unhealthy. Whether you feel "fine" or not isn't the point, it is still unhealthy and can have long term consequences.
Although 58 bpm resting is pretty dang good, that's a runner's BPM to be sure. I remember when my best friend used to run track, he would have a resting BPM of 44!
Taken off of worldrecordacademy:
Daniel Green achieved the world record for the slowest heartbeat in a healthy human with a heart rate of just 26 bpm