I have had numerous experiences similar to this... I am pretty sure the paralysis is just Sleep Paralysis. Supposedly when you fall asleep, hormones or something are released that "paralyze" the body to prevent it from acting out dreams and such -- or -- the part of the brain that controls bodily/motor function shuts down ... I'm not exactly sure which is accurate, but you get the idea. It's basically the opposite of sleepwalking, which is caused by the motor functions of the body *NOT* shutting down, and thus able to function while in a dreaming state. The experience of Sleep Paralysis comes when you gain awareness ... or a part of your brain "wakes up" ... in this state of being in between dreaming and waking, before the paralyzation "wears off"...
Waking up in such a state can be pretty frightening itself, but there's often more...
For me at least, and it sounds like for you also, there is usually some kind of "constant" sound during the experience. For me it's usually a high pitched vibrational ringing/buzzing (hard to describe exactly), but there have been times where I've heard loud "roaring wind" type sounds... like a jet or hurricane was around me... very loud, somewhat violent "windy" sounds. There is also often a tingling sensation that accompanies the ringing/buzzing, especially around the back of the neck/head and upper back/shouders area. I guess this might be what you're referring to as "euphoria", but I never really thought of it as that.
I've noticed these experiences happen more often if I had been trying to practice some kind of meditation/awareness technique while going to sleep... like, while trying to actually stay awake and maintain awareness, but ending up drifting to sleep anyway. That "effort" for awareness I guess sometimes causes awareness to "pop back" after the sleeping process has already begun ... leaving me in this in-between state...somewhere in between waking and dreaming, unable to move my body, hearing buzzing/ringing, and feeling tingling sensations. Often there are beginnings of what could be considered lucid visuals... But I have never really "let go" and gone very deep into the experience, because usually when it happens I feel a sense of like, a presence of some kind possibly being in the room, and a feeling of helplessness (due to being paralyzed). Due to this feeling, it becomes urgent for me to try and open my eyes and look around or make some kind of noise to snap out of it, to make sure there's nobody there. Sometimes it takes many seconds (how many, I have no idea) of "trying" until the paralyzation finally wears off.
After the fact, I almost always regret aborting the experience, because obviously there's nobody else there and I realize the "presence" feeling is mostly just due to the fear of possibly being attacked while in that completely paralyzed state. Every time it happens, I always feel like that "in between" state is a doorway of sorts, and that if I were to dive into it there would be interesting things to discover, but when you're *IN* it and it's happening, it's almost like being on a drug or something... thinking processes aren't the same as if you were wide awake, and you don't always remember to just ignore the fear, so more often than not I end up aborting.
Little by little I've gotten better at maintaining composure and awareness during all this, better able to not worry so much about being in that "helpless" state, and to just focus more on the internal experience, to "enter" into the ringing and tingling and find out what's on the other side. It's almost like being afraid of a dark pool of water, and slowly overcoming that and dipping your toes into the water, but still not "letting go" completely and just diving in...
Anyway, what you described sounds pretty much like the same thing I've experienced a lot, so I figured I would share some of my experiences with it too, and see if any of it sounds familiar to you. What I've explained is just the "common basics" though... often the individual experiences have varying degrees of lucid dreaming, imagery, audible sounds, etc... even outright hallucinations... but the "basics" always seem to be there -- paralyzation, ringing/buzzing of varying pitch and intensity (sometimes a strong or even violent "wind" or roaring noise instead of ringing), and a tingling sensation that I always seem to associate with "light"...
--James