From what I've heard on here, its extremely normal for a 4.5 - 5 month old to dramatically change their temperment as well as sleeping patterns.
That explains why I hardly recognise my son at the moment... he is just such a different baby to the one I had gotten to know up until this point in time.
The brain in overdrive makes a lot of sense - but it doesn't help stop me feeling like I'm doing something wrong at the moment. I just can't seem to make him happy for more than a short period of time.