Thanks Imsmum!
That's interesting, I've been reading online and some sites say to wake the kid up and "snap" them out of it, but most agree with what you've said, as comforting can just add confusion and make it worse.
I'll try and leave him, but I did that last night and it went on for over 5 minutes... seemed like hours though!!
Last night he did it at around midnight and then a minor wake-up, which was helped by comforting around 2am...
Also, in my reading, they can start as early as 9 months.
I wonder what Tracy would say about this?