While in the Navy I was once required to be awake for 172 hours. I don't know about sleep deprivation being fatal but I do know that without supervision your actions can cause your death or the death of others. Most people can go 24hrs without to much problem, and allot can go 48 without to many problems, but after that your mind will start to wonder and it's almost impossible to keep you mind on all but the most demanding situations. After 3-4 days without lots of outside stimulus you will sleep sitting standing doesn’t matter, but never fear when you fall it usually wakes you up. I would like to tell you what it was like after the 4th day but I honestly can't, I remember small bits like you remember a half forgotten dream from the last few days. People yelling for me to wake up, banging my knees and other body parts as I passed out and fell. Trying to perform do complex jobs and not understanding what the tool in my hand was for. From my experience in a survival situation an untrained person can stay awake and still be able to perform reasonably well for about 3 days maybe 4 max. after that your mind can't be trusted. You may in a life or death situation be able to go longer but if there is any time at all even a short 10 min nap will help.