Spot on Capt PB
You have described precisely what happened to me this week.
I may only do 4 regional sectors per duty but, doing a bunch of earlies followed by a late messes with the body.
On the day of my late duty I woke up at 04:30 and never managed to get back to sleep! On the third sector of the late duty I (HP) made a mess of an approach and ended up flying a less than average visual. I was tired, but only then did I realise how tired. I was glad the next day was a day off.
Its not the total Duty Hours, its when you work them. CAP 371 does not give rostering any guidance in this area, it simply gives limits.