If you were very young it was probably just an intense out-of-body experience during sleep.. used to have them all the time when I was young, up until about the age of 14/15. If you were born 200 years ago it wouldn't be aliens but demons or whatever cultural boogeyman was in fashion at the time.
However, that is not to say the entire thing is a total figment. I'm of the opinion that entities or intelligent things can and do mess with people in the sleep state, but we lack the appropriate mental apparatus to perceive them as they really are.. or they may even be essentially without any real form.. either way what we do is mental projection by overlaying our own imagery on top of these things, then witness our own subsequent projections.
I've seen UFO's, but they weren't silver discs or anything like that. They were orange balls of light that sort of faded into view, zipped forward or zagged a bit, with what looked like little sparks emanating from them, before fading out. Sort of like shooting stars, but slower, larger, and probably about 100-200m high. This happened once post-NN DMT, and a few times when I was completely sober but I was in an emotional state.. once after watching Good Will Hunting actually, it put me in a very strange mood. What was strange about those times was that there seemed to be some sort of mental connection between myself and when these things appeared, very difficult to describe.