My seemingly perfectly healthy and well-cared for (micro chips, shots, regular vet check-ups, topical flea drops etc etc) 5 month old male kitten has started doing this, but his brother hasn't. I agree, it isn't the same thing as the twitch when they're falling alseep (myoclonic spasm/hypnic jerk), nor is it dreaming kitty twitches as they chase after that mouse in their dreams.
This, as the poster says, is different. It's a persistent, on/off shivering seperate from purring, that occurs WHILE he's falling asleep. You can tell when a cat is falling asleep as their purr fades and cuts in and out like an engine running out of gas, then stops as they fall completely asleep.
The shivering is whole body but not all at once, so his shoulders/head will shiver, or his haunches will shiver. They sometimes both shiver, but there's usually a switch. It happens on whichever side is uppermost, so I guess it's going on on the other side too.
Having read up on it, and being fairly sure that he's a basically very healthy little guy, I think it's either a touch of low blood sugar - he's not too keen on the new food we got - or he's a bit cold. I've made sure he's eaten something and drunk some water, and popped him on a big duvet.
If the shivering persists, it's vet time, as it could be a reaction to something he's nibbled, a fever, or something awry with his kidneys, but I