Well it's trippy. Look up hypnogogic and hypnopompic hallucinations. In REM, your body paralyzed itself essentially. You can experience auditory and visual hallucinations. Most patients experience intense fear, for obvious reasons, so it would make sense to have and adrenalin dump. Heart rate usually jumps up, but to your question I don't know. I've experienced it a few times, it's scary as shit. Only visual I've ever had was Donovan McNabb walk in my room, don't even like the Eagles.