Unfortunately I can't remember which book this came from, it might have been "Raising Your Spirited Child" (terrible name, great book), but one of the most useful things I have ever read in a parenting book was to think of your child as a growing pyramid of blocks. Every 6 months or so they get another block and in order to have a well designed pyramid they can't just plonk that block on the top of the current pyramid, they have to knock it down and start again with all of the blocks. When they are going through a stage of "up-skilling" and re-integration expect outbursts. I felt like it really explained my kids' tendency to track along beautifully for a number of months and then fall apart for a while and then get back on track.
I think that was Raising your spirited child because I read it too. I found it really useful too. It reminded me of when they were babies and they woul have a week of disturbed sleep, then suddnly start doing somthing new, like crawling for example. Sometimes it isn't that obvious, but I definately find my hardest times coincide with developmental leaps. These stages are AWFUL when you are in them, but they do pass!
Another possibility might be a sleep debt. When people miss heaps of sleep, it can take a xery, very long time to pay back that debt. Again, the only cure for that is time and sleep.
