I feel way better when I work fasted. I love not feeling heavy or weighed down ... Not eating a high carb diet also reduces brain fog and general feelings of lethargia (ie panini and gigantic cinnamon bun for lunch)
Well put!! Though there's a bit of a "tricky" caveat to this, or 'tricky' if you don't understand the mechanisms at least which you won't if you haven't read-up on fasting/ketosis....When I go days, weeks even, w/o ever having a calorie before late-afternoon or even dinner/end-of-day (my most-common day sees me getting 90%+, if not 100%, of my calories in 1 sitting, right-before I fall asleep or rather before / as I'm getting in-bed & turning-on a movie, sometimes asleep immediately after eating othertimes an hour or 2 later), it does tend to make me run smoother/better while working although I find that I can "break" the routine by using low-dose gatorade(or similar) products through the day IF they're spaced-out (this basically negates the brain-fog and lethargy you mention, since those are attributable to the quick spike in blood sugar & corresponding insulin-spike which, once it's run its short course, leaves you lethargic/foggy!) For volume/amount I mean like a 1l gatorade that I'll drink sparingly over a multi-hour period, maybe every 3rd time I go for fluid I'll have the gatorade over the water (always go through at least 1-1.5 units of water from my 2l, insulated steel water bottle, thing is a beast it clips to my belt I got it after a time that I was at-height and dehydration hit me hard, both psychologically and even worse in terms of cramping which led to a generalized anxiety/panic about being at-height in that condition (I'm in semi-tropical central FL!)
Should mention how critical electrolytes are!!!! When I prep my water for the day, it's not just neat/plain water it's got a good dash of sodium-salt and equivalent dash of "Nu Salt" (potassium), or just the "Lite Salt" which is a 50/50 sodium/potassium mix, all of these are Morton products and work great for hydration, I spent about 4yrs here commuting to jobs by bicycle so had plenty of trial&error learning to finally dial-in hydration, the half-half sodium/potassium (and Magnesium pills, as dosages for that don't lend well tttowards powders - never get Mag Oxide always Mag citrate or any form besides Oxide, Oxide has pitiful bioavailability) makes all the difference in the world, people get dehydrated working outttside all day in the summer and if/when they hydrate with water alone, sure, it gets rid of the dehydration but it exacerbates electrolyte-deficiency, before I figured this out I would think I was hydrating appropriately only to find I couldn't figure-out why, after getting cramps and then drinking a half-litre of water, the dry-mouth would go away but the cramping would get worse sometimes to debilitating cramping/charlie-horse type muscle-restrictions/tensions....When you've sweated-out a liter of fluid, you sweat-out the corresponding electrolytes, so while drinking neat/pure water addresses the dehydration it actually exacerbates the electrolyte-deficiency as you're watering-down the already-depleted remaining electrolyte reserves, Morton's Lite Salt - just a couple dashes into a 2l bottle - seems to do the trick (ideally alongside a 250mg mag citrate pill though I find that magnesium-deficiencies don't manifest remotely as quick&significantly as potass./sodium deficiencies)
It's also especially nice in the summer for your water and electrolyte intake to go directly to your brain and muscles instead of being redirected towards water retention in the cells due to carbs
Full-bellies and all? I don't know if you mean intra, or inter, cellular water-retention, one is good (and is a primary mechanism by which the outstanding supplement Creatine works so well!!) and the other is bad, however it's not just that in fact I'd wager the bigger water-loss from carbing-up while hydrating is simply due to the water necessary for digestion (your body can't digest properly when dehydrated so if you had a liter of water, but also a bag of frosted donuts, you may've just had a "balanced hydration snack" but if you were already dehydrated then that means it does nothing for you (as you didn't need "balance" since your current position was sub-optimal, you needed a boost in water but if it went entirely to digesting the carbs then you never changed your water-level in terms of proper hydration you merely didn't make it worse by having that snack - and then, if the snack wasn't appropriate, you compound things by getting the carb-heavy results you'd mentioned *on top of* your dehydrated (and/or electrolyte-depleted) state!)
Totally a balancing-act much of the time, have spent years trying to dial-in hydration and found nothing that's superior to simply keeping ample water-with-Lite Salt(or gatorade, preferably diluted with water) for negating hydration-issues, and while heavy carbs (or calories of any sort) are likely to cause rebounding efffects while working, IF you just do small-doses of carbs with heavy fluid I find it gives virtually no 'carb zonking' afterward, I've gotta surmise it's because your small carb-intakes aren't enough for any significant blood sugar/insulin responses, yet *is* enough to give a lil boost of blood glucose to help you perform a bit better (almost like low-dose caffeine)
My favourite days of work involve massive takedowns, 8-9 hours in tree with just water and salt tablets
What type do you use? They've always intrigued me but honeslty it's so simple to do a dash of Lite Salt, always pop a 250mg magnesium citrate or two before heading out, and it's never steered me wrong

Now I just gotta get gigs where I'm able to spend 8-9hrs in a tree ROFL (only recently started canvasing the locals to get "high up on their contract-climber roster", actually just got my 1st job that way last week it was great it was the 1st time ever I got to just climb/rig/cut and didn't have to do any pick-up, didn't load their trailer, just sat in-tree roping & cutting it was ideal...exceptt for the pay, although at this point I'm just trying to get more time in-tree / not stressing the $$ as long as I'm not taking that much of a loss relative to what I usually make doing my "general handyman" work (have done that for about 5yrs now and only this year did I begin trying to specialize, with my final completion of my heavy-duty rigging setup just barely a month ago, so far have only reached-out to 2 companies so was stoked 1 contracted me within a week of me telling them to keep me in-mind ;D )