Why does this happen whilst falling asleep?

I'm just about to go bed after surviving an all nighter the previous night, so I'm really tired. My neck also really hurts, but I SERIOUSLY doubt it has any relevance. I lie down, begin to get comfy and I experience the sensation that occurs when you lie down, and your body suddenly jolts like you're falling. I know what this is, it's a hypnagogic jerk, I get them all the time. I remain still, dozing off and then just suddenly, I go into some weird stage where everything seems a bit louder, my heart starts beating much faster and I can't move my body. For some odd reason, this really scared me whilst I was in the state, and I started shouting but my mouth wouldn't move, but I could hear myself shouting? I don't even know. My only choice is to just close my eyes and try to get to sleep, but then everyone goes "weird" and I wake myself up. I'm actually anxious about trying to go back to sleep again. Why is this happening?