Most of the time I don't remember my dreams. I close my eyes and the next time I open them it's morning and I have to get ready for work.
Every now and then though that cycle gets thrown off, I'm not sure what exactly causes it, but sometimes as I'm falling asleep, I'll jolt awake, my heart pounding and eyes wide awake.
I'm not really sure what causes these feelings of falling but one time I read a really interesting book with a theory about the cause of these abrupt awakenings.
Basically our being usually just floats around in our mind, doing its thing, when all of a sudden it will encounter a step to another space. As your being starts climbing these steps to other locations, each step gets more and more precarious. Becoming so wobbly that you inevitably fall. While falling, you could witness anything from future or past events, other worlds, etc. Usually these memories are erased upon waking up, but sometimes they are left to bounce around our subconscious.
How do you think the feeling of déjà vu came to be? This can also explain a child's immense imagination. They are more likely to hold on to the scenes they see at night because they are more likely to believe. An average adult may not believe that there is a full civilization underground, but a child would take that thought and build on it, blurring the truth until no one could tell what was originally seen and what was added.
When I was a kid I wondered if maybe that feeling of free falling was actually me trying to run from something in my dreams, if I had failed to live in my dreams and was forced back to wakefulness.