I get this too. While awake, the sounds I hear on my own mind are just basically the signs I could make with my own voice. I can imagine a song by imagining humming or singing it, but I cannot recall what the actual recording sounds like. Yet there's that switch that flips as I go to sleep that I notice occasionally where I can hear crystal clear music. It's beautiful and I can't recognize the music.

I also get this, much more rarely, with images. While awake, I can't truly picture what I imagine, it's like I just have a notion of what is there, like motion and shape, without really seeing it, or at best a momentary flash of a picture. But when drifting to sleep there is a moment where the images become real, with color and texture and detail far beyond what I can imagine while awake. It's shocking and wakes we up when I notice it happening.

What is this switch? How can my brain have such discretely distinct modes of operation? Falling asleep itself seems so much more of a continuum.

A similar thing happened to me after some sleep deprivation in a noisy bar. I was moving to the music, and someone said the music had stopped. I listened while still moving to it, saying no it hasn't. They repeated that it had. I listened again and indeed there was no music in the air, only my mind. Cognition and perception is based on hallucination around stimuli--sometimes we fill in more than the gaps.

I've had this a few times. It's like listening to a good album on your headphones, except you are in control of the music. Being able to do that was a wonderful experience, especially since I've never actually played an instrument.

If you try to sleep with your eyes partially closed, you’ll start to hallucinate the things in front of you. Things become shapes, and then completely different things. It’s pretty interesting.

Something similar happens only when sitting on the couch and watching something on the TV.

If i am tired enough, then i can fall asleep, but am somehow also able to hear myself snoring.

Dreams do not happen in this state.