How can my dream end with something that I end up actually doing?

Not sure exactly how to word it, but why do my dreams sometimes end with me actually doing that action? For example, I’ve had nightmares in the past where I scream at the end of the nightmare and in real life I actually scream. I just woke up from a super vivid, almost lucid dream where I spit in a trash can right as I woke and could feel myself spitting (not drool). Doesn’t happen often at all and only when my dreams are super vivid. Just wondering TIA