One night I was going to bed and so I layed down, and suddenly my heart rate started to slow down like crazy, and each heart beat felt like it was pounding really heavily inside my chest. I was scared so I stopped laying down and sat upright, and it quickly went back to normal. When I layed back down it started doing the same thing. My heart rate is normally 80-90 bpm and during this moment I would say my heartrate was at about 40-50 bpm. I think this lasted about five to ten minutes. Does anyone know possible reasons as to why this happened?