Lowest heart rate?

I slept with my Apple Watch last night and I didn’t know the Apple watch’s lowest heart rate it will record is 30 bpm. While sleeping, it looks like my heart rate may have gone into the 20s in the middle of the night since the graph shows me flatlining at 30 bpm most of the night. Is there any other apps for the watch that will record below 30 bpm or is it just a hardware constraint?