Sitting comfortably in your harness and gently pulling on the brakes now and then, flying does not seem to demand a great effort. You are not running out of breath or sweating excessively, and your muscles are not aching. And yet it does. Since it is a mental effort rather than physical, this is not obvious and therefore often neglected. Until it is too late.
Pilots are travelling in a dimension they were not designed for. Unlike birds, you can not rely on natural reflexes. You have to think to fly. And thinking requires energy. Training can lessen this handicap (Remember your first driving lesson?), but it will never completely disappear. Flying might become a second nature when you are paragliding hundreds of hours a year. But even then, an eight hour flight will generally be far more exhausting than an eight hour car ride.
Because you are spending more time analysing and thinking, vol bivouac demands more energy than an average flight. And if flying does not make you tired, then hiking will. No matter how good your physical condition, eventually you will be running out of energy. Your body becomes tired and needs to recuperate. Resting for half an hour and eating and drinking a little, can give your body a boost. I have found out that by resting every two hours or so, I am more likely to stay out of trouble.
If you are exhausted rather than tired, then a few hours sleep will work wonders. If you continue nonetheless, your body has not enough resources left for all of its participants and starts to ration the supply. Essential parts like lungs and heart will get most of the (little) resources that remain. Other parts get just enough to survive or get switched off temporarily. Your body is entering survival mode. That is not a pretty mode to be in. Better stay out of it and rest for a few hours.
The human brain consumes a lot of energy, but strictly speaking is not necessary for moving around. My experiences is that when your energy level gets low, your body still goes through the motions but essentially has switched of its brain. You can still steer your glider by moving your arms (but possibly forgetting to shift your body as well) and put one foot in front of the other in order to walk, but the reasons for doing so are not clear. You might be steering or walking in the wrong direction, or giving the wrong inputs during a big collapse.
Your thinking and decision capacity is (severely) limited when you are (very) tired. This is likely to result in (costly, possibly fatal) mistakes that you could have been avoided if you would have been as fresh as morning dew. And when you are really tired, you are essentially brain-dead. Why are your still flying or hiking at this stage?
I am not a doctor, nor do I have any education in the medical sector to back up my opinion. But even though I could be wrong about the causes, I am sure about the consequences. I have had just too many incidents where I was still hiking or flying, while my observational, cognitive and decision skills were (severely) limited or even non-existent. This resulted in (unnecessary) stress, large detours, air acrobatics, hours of extra hiking, and even injury. And it is not just me. I have seen it with a lot of other pilots as well.
Being (too) tired is a major killer in paragliding and hiking. No matter what you do and no matter what excellent physical condition you (used to) have, eventually there comes a time when your brain simply stops making logical and adequate decisions. For most of us that means it stops working. It is still there, but it is dead. You might still seem in control, but this is no more than outward appearance that only you believe in. Outsiders will be quite able to see that you are exhausted and reached the amoeba stage. The stage where you are moving, but not thinking. You are still flying, but you do not notice that you have become nothing more than a bag of sand in your harness. Like those bags of sand used for adjusting the centre of mass in sailplanes. Did you ever wonder why those are called deadweight? Now you know.
Evolution gave us heavy bones and body, as well as a comparatively large (and thus heavy) brain. It seems we wanted to think, and got want we wanted. Birds wanted to fly, so they mostly got rid of their weight. A considerable effort for a dinosaur. In the process they lost most of their brain, just to be light enough to fly. I am not suggesting you should strive for a birdbrain in your quest for flight. I am only asking you to be aware of the handicaps humans face when trying to fly by themselves. And not to fly if you are not.