Answered

Your heart beat usually _________ when you are asleep

ADoes not change
BSpeeds up
CSlows down
DStops

Verified Answer :

C.slows down beacuse enough are not doing anything so your heart slows down

I would say C, slows down.
I say that because while sleeping, your brain AND body are asleep, so everything in your body is relaxing, Including your heart.
Hope this helps :)