I was trying to sleep and ended up falling asleep without knowing it. Well in the dream I was still awake in my bed, trying to fall asleep so I went to my computer. The room was dark and I looked over and sensed there was a malevolent entity in the room. I felt the adrenaline rush of a chill that runs down your neck and into your spine when you are terrified and it was realistic. I also felt like I was gonna pass out and started to fall from my chair. This is the point where I woke up.

Also, a few weeks ago I had a nightmare so terrible I began screaming in my dream and woke my parents up. They came running in to see if I was ok and I couldn't remember a damn thing about the dream.

So these are the questions I have:
How could a feeling in a dream seem so real?
Can a dream be so terrifying that when you wake, your mind blocks it completely out?