My Baby Is 4 Months Old I Had Him At

My baby is 4 months old I had him at 27 weeks he just got home from the hospital on the 30th, he's on oxygen and 2 monitors for his heart rate and oxygen, but I'm very concern because his heart rate drops to 100 but it just dropped to 75, but only drops when he is sleep. i can't sleep because I'm scared/nervous something may happen, is it normal or what to do?