Not sure but;
Heart (the body part)
Each morning, I am given life by my master
Hearts return to your normal heartbeat and the "master" here is a person.
I work for my master all day long, and sometimes get heatstroke
Hearts always heartbeat, and the heart always suffers from damage because it did not cool itself. That is why you get heatstroke.
But, after a nap, I am back to work
If the person naps, Hearts will only have less heart rate than usual, but when they wake up again, Hearts turn back to their average heart rate.
At night, before bed
My master kills me
When people sleep, heart rate lowers
Only to wake me again in the morn
They are back to their original heartrate.