Ya, amount of physical activity makes a difference. Also your environment and diet. I have a field job that involves a lot of driving in rual areas and such while I live in the big, bad city. I find I can operate safely for days on approximately 4 hours sleep /day, in 1 to 2 hour cat naps, as long as I don't come back to the city. 5-6 hours/day average and I can go 7 days/week for weeks. I'll camp or sleep in the vehicle often, bascially when I feel that alertness could be suffering (am constantly aware for this as it is my biggest safety risk). Sometimes I get a motel room, but usually that would cause me to have less sleep because I have to drive there. Besides too much can 'soften' you up, and then you have to convince your body to recommit. By this I'm refering to the second worst part of it all, getting your body to resign itself to the grind, which can take a couple days.
The worst part though is stopping. Your body experiences such a letdown after running full out that you are quite vulnerable to falling sick from cold/flu within a 48 hour window. You get back to a super comfy bed and you are driven to sleep. While usually not physically demanding, the job is often mentally stressful, if for no other reason than having to will yourself not to just quit and drive home on bad days after a month non-stop work. Unfortunately while you sleep your body drastically lowers your immune system functions while goes through the body renual/repair cycle.
I've found that diet is key while working. Heavy on fruits and veggies, little or no resturaunt meals or processed snack foods (pop, candy, and especially chips because of the flavour coatings). Protien from nuts/seeds, but not including peanuts since even unsalted dry roasted peanuts ones make me thirsty so I take that as a bad sign, and the occational hot meal of an omlette. Also I take a daily fish oil supplement, but that is to nulify mild depression bouts I have historically had dating back to a nasty divorce/custody battle years ago. Unless it's very hot weather, or I excert myself a lot, I eat enough fruit & veggies that I don't need to drink. That all coupled with a Cold FX regimine when stopping work seems to do the trick of keeping the body clicking right, remaining safetly alert while living on cat naps, and not falling apart at the start of downtime.
However as soon as I drive back into the city "bang" it's back to 7+ hours/night. I think it's the lower O2 content, or the much higher CO and ongoing air polutant content (the city is not particularly bad, but you notice it coming back to it). Also if I get into a situation where I physically tire myself, such as getting stuck damn good in a snow drift where I have to shovel for an hour straight, or my job turns towards the physical (cleaning drilling mud tanks or digging a lot of soil samples). Then the sleepies creep up and I need some extra sleep for the day, sometimes right away if I had been going for a lot of hours straight since napping.
