For me, in addition to the nesting, I just plan on being worthless and panic-stricken for the first few days. I try hard to prep at the end of one term for the start of the next to make the return a bit easier.
I find that after a few days of struggling, I get back into the mental groove and seem to have fewer mental cobwebs.
Caffeine is a must. Small rewards for small progress are great. And loading my to-do list with every single thing I have to remember give me some sense that I am making progress as I cross even trivial things off.
I find the idea of returning to teaching a little bit daunting. Once I'm in the classroom, though, I'm always thrilled to be back. Teaching really normalizes me and gives me a purpose. Working on the diss is just too nebulous and unframed of a process, though I'm closing in on it and should defend in 2 months or so.
I will say that this semester is off to the rockiest start in years due to things totally unrelated to academia. I feel like I'm shouldering a ton of non-job/non-academic stress and it's really distracting. I'm part of a "two body problem" so right now I'm haunted by the fact that just 3 weeks ago, I had very few problems (well, they were about to happen), my house was full and wonderfully decorated for xmas and we were going to party after party. It's odd how radically different early January is. It's cold, dull and honestly, really depressing. The house is quiet and uncertainties in my life are looming large. The bacardi next to me is helping take the edge off (a bit). I miss my spouse a lot right now since another someone in the house is better (sometimes) than no one.
Good luck to all.