What's this sensation I get sometimes when I'm sleeping?

I’m generally a lucid dreamer, so maybe it has to do with that? But I’ve noticed that for a while now I get these really weird sensations when I’m somewhat aware while sleeping. It’s not sleep paralysis; I know what that feels like. In this sensation, I literally feel my heartbeat slowing down. I always get scared because I feel like I’m going to die if I relax and let it happen, so I try to keep myself aware and my heartbeat goes up but then slows as I fall into a deeper sleep. Now that I’m completely conscious, I know it’s nothing to worry about. But when it’s happening I’m still in a slight dreamy consciousness and I always think something bad is happening to me. It’s not a nightmare either. I know I’m at my bed and everything. I want to know if other people have had that sensation and if it has a name.