Something I've read over and over in the last almost-three years that I've been a parent is that babies' and toddlers' sleep is very indicative of their development. When their sleep becomes disturbed (lots of nightwaking, needing mama, needing help getting settled back down, etc.), it's because they're in some sort of developmental phase. It might be physical (crawling, walking) or cognitive (preceding a language jump, a shift in perception, etc.). Parents tend to freak out and try to get these phases to stop, but if you just go with them and know that they're important and developmentally normal, and respond supportively, they'll resolve themselves.
And the thing is, in my experience this is totally true. They last a few weeks, and just when I've gotten used to waking up several times a night to a kiddo who wants to nurse or convince me to get up and play, it stops. He suddenly sleeps solidly through the night again, and there's always some amazing shift in what he can say and do. We've just come out of one of those phases, and for the last week Carter's been sleeping soundly again. The first two nights, I actually woke up periodically to make sure he was breathing!
What seems to have changed this time is a shift in the ways he understands number. Before he was counting sort of haphazardly, and though he understood the idea of one-to-one correspondence and would point to things when he counted, he didn't keep track of what he counted. He would count the same set of objects and get a different number every time, and would count the same object multiple times. Now he has some strategies for keeping track of what he's counted, and when the number of objects is less than ten, gets it right a lot of the time. He can also look at a set of two or three objects and immediately identify its number without having to count.
Tonight he had two toothbrushes (he likes to brush with several at once), and said, "Carter has two toothbrushes." Then he saw a third one on the counter and pointed to it and said, "I want THREE toothbrushes!" It was interesting to me that he did this without counting, so he seems to understand that 2+1=3.
There are other things in his language that have subtly changed. He's using verb tenses and pronouns correctly more and more often, and his sentences are increasingly longer and more complicated. I recently watched some videos of him from a year ago, and it was interesting to see how different his language use was then. He was still signing a lot when he talked, and his sentences were mostly two-word structures. It's amazing how much they learn in a year!