Recently I've been trying to create a sub-race of human species that's more efficient in... Well, war. Mostly stronger & more efficient, and because of that I want to ask a question. What "modifications" a human body would need to be able to sustain itself and be able to work normally without food or sleep (or even water) for lengthy (longer than average human's) periods of time?
Ability to control homeostasis helps so that in warm temperatures the body doesn't need to waste calories keeping itself warm. Cold-blooded animals need to eat much less frequently than warm blooded animals, but their activity level is also controlled by the ambient temperature.
Desert animal physiology, and even birds, are also much more frugal about water consumption. No urination as we humans recognize it. Human physiology is wasteful with water in order to achieve superior cooling but you could mix this with the above.
I think it would be easier to make them able to eat and drink almost anything, but you could just give them greater fat deposits but there are probably limitations to that unless you give them a fatty tail like some lizards have. But that would encumber combat since you always have to carry your reserve calories with you.
Sleep is tough. Some people are genetically pre-disposed as much more efficient sleepers and are bright and bushy with only 3 or 4 hours of sleep per night. You could use that. Alternatively, you could assume that it's possible to just engineer sleep out since we don't fully understand the mechanisms or need for sleep.
But if you do assume it is fundamentally required then you would need some rather impractical solutions like double-brains alternating but that obviously has huge problems such as energy costs and a split individuality and skills. You almost might as well just have two people alternating sleep duty.
Try something with photosynthesis. The species would probably look like a sort of plant/human hybrid but imagine how cool would it be to have a solar powered creature. You can make them live in a polar zone where they hibernate during the dark winter to balance it out a bit, since nothing can survive without a recharge. They wake up in the summer charged with constant daylight and go off to ravage southern lands. That’s just an idea. See what fits your setting.
Let's tackle the two parts -
- Without Food
It's not too hard to live without food for an extended period. There are many mammals on earth that already do that - for ex. lions eat just two to three times a week. Our bodies might already capable of sustaining on such a diet - there are still some isolated tribes who might not eat for a few days if they fail to catch a prey.
- Without Sleep
This is a bit tricky because we still do not understand the complete science of sleep. Apart from the psychological aspect of sleep, there are also skeleto-muscular effects. During sleep our muscles are completely relieved and it promotes muscle building and recovery.
Thus, for sleep, although there might be a way to prevent it, the easiest way could be just reducing it maybe even to a few minutes. To make sure all the advantages associated with sleep happen in this short burst, we will just need to ensure that the same hormones are released - just in greater quantities and ideally with a catalyst which can speed up the process.
The more we can accelerate the work that the body does during sleep, the shorter the process of sleeping can be.