I had them as a child and it was whenever I had a temp. I'd wake up screaming and screaming and there was no way of snapping me out of it, and I wouldn't remember it happening when I finally woke up - snapped out of it. Excpet one time I remember feeling really really overwhelmed and there were number and graphs getting bigger and bigger and I felt overwhelmed by it all. Weird. I grew out of them - thank goodness! I remember my mum having to see my teacher and telling her all about it incase I fell asleep in class.
My DD has had a couple crying ones which I believe are like terrors, and it would go for ages - I remember timing one and it was 40 mins, continual crying and crying that would not stop - so draining. I did find with her last one singing softly helped. And I have tried clicking quietly like in a slow one and two... to try and get her to concentrate on the sound.