We are not aware of falling asleep. We usually don’t realize it has happened until we wake up. Sometimes we lay in bed for a while, feeling the difference between dreaming and wakefulness.
Sometimes we become aware of the dream while in it. And then we either wake up gently, or immediately, or just keep dreaming.
Sometimes, after waking up to the dream, if we do choose to stay in it, our consciousness drifts again, and we forget that we have not woken up.
Dreams feel so real while we’re in them. I’d like to say I can tell the difference – that I know for sure when I am awake.
But I might be wrong.
This Will Be Changing 
There’s a new show on TV about this, called, appropriately enough, Awake. It is an interesting premise. So far, I am enjoying the show, but wondering where it will go.