I think a lot of that depends on the particulars of the project at hand and all the other people there as well as the budget.
If we have what we need, then there's a lot less worry then if we need to figure out how to cover a scene without the right lighting or ability to light.
A lot also comes down to the producers and directors you're working with and how well you seem to get along, it can sometimes be adversarial and there is no getting around the politics of filmmaking.
Personally, Sometimes. It's not a restless sleeplessness, more that I'll get an idea and then a chain of ides which'll keep me up thinking them through.
I tend not to look at movies for inspiration, and I tend not to like "reference films," personally, but of course there are many times when that's the form of communication with the director, and so you do have to watch them all.
I never stop thinking about the film(s) I have in varying stages, but I tend to focus most on the one next at hand. It's not an all day everday kind of thing as life on the whole throws things which distract you but I'll be the most "in tune" with whatever film is next, until and unless something more pressing appears which requires more immediate attention.
Generally I'm much more concerned with speaking with the director on the script, story and their ideas of how they want to convey it. Giving them my own ideas which might be better (or worse!) and figuring out the over-all plan for the movie and what the movie is trying to say. I'm concerned about the budget, how much we have to play with, and then I'll start thinking about the locations, and what challenges they'll bring. They'll be testing, if need be, especially if doing something new, or something you haven't done in awhile, to make sure it fits into the whole pipeline through post, lots of conversations with production design, wardrobe, makeup, gaffers, camera ops if needed, and the key grip and the ad and everyone else as we get closer to shooting, what issues the locations will bring up etc.
Even when I'm not 'officially prepping,' e..g off of the clock, (ha!) i'm looking into things which have been brought up which I don't have as good a handle on as I should, or new things. Examples recently were Rat-Packs / DMX as I had a few shows which used them, and I didn't really understand them, so I went deep into reading all I could about them in my off time..
Just now I finished reading a script for a project I haven't even been hired on yet, and I'm starting to think about how I would approach some of the car stuff at night, where might we use a cool drone (and can we do a day for night on a drone? and VFX in car head-lights?) I might make some doodles of my thoughts and the printed copy of my script is covered in green ink all over with notes, ideas, lights we might need, lenses I'd like, diffusion filters I think would work, aspect ratios, as well as my own inner dialogues-- writing out with arrows my thought processes. And I haven't been officially hired! and may not be.
Tomorrow I go to a prep meeting for a short I'm working the end of the month. We've done over-heads and shot lists already, had the talk on budget and things we'll need, but tomorrow we'll talk scheduling (as there's a conflict) so tomorrow I'll be going over everything we're doing end of the month with the AD, director, producer, trying to figure that all out-- but I'm sure i'll also be thinking about what I want to make for dinner. Is my Fiance ok, I hope she's not mad I'm taking longer than I said I would (she won't be), oh my god I need to get gas in the car and I'm on the west-side and it costs too much here etc.