First Note: I was in Boy Scouts. Yes, I know, now it's evil, But I'm proud of it, to those detractors; while I now disagree w/ many of their policies since I'm older and 'get it', I still am happy to have had the chance to be part of it; if it helps, our scout troop must have been in willful violation of the rules, as we had some alternative lifestyle scouts, and a female scoutmaster, and it was never an issue, so YMMHV.
On to the story: My final year at camp was a bit of a goof-trip, since I'd already made eagle, had more badges than I needed, and I was going to be taking swimming, rowing and canoeing, and doing the mile swim, for example. None of these were really mentally taxing, to say the least, but they *were* going to be relatively physically demanding. Since the other guys in the senior patrol were geeks as well, we decided to make an endurance test of it. I was the chosen test subject, with each of them volunteering to help. I stayed awake the entire week, while completing my badges and the swim, with a different guy 'on shift' each night to help observe the behavior of someone going thru all the loopy stages of being awake for days. We played roll-playing games, listened to music, worked on paperwork for the troop, fished for raccoons, you name it. I made it through, and turned out thru a serious of drop-offs to be the last man home. My folks were at the college campus where my older brother was doing a summer program. I walked in the front door of the house, made it a few steps inside, and passed out :) I woke up a few minutes later, sore from the pack crushing me into the carpet, dragged my tail to the basement to chuck laundry in and then fell into the tub, passed out till the cold water woke me up, made a sammich, ate it, and fell into bed. I was up and at 'em the next morning, and back to my usual schedule, after almost 8 days w/o sleep.
Effects I observed: Obviously, at first you get really, really tired. Then you get cranky. We jokingly referred to this as the toddler stage. That said, once you get thru it, you start to get that second wind, and you're fairly wired. A few days in, you CAN start to hear odd things. Not sounds, ghosts, etc, but sometimes what people say isn't what you heard, and it requires some repetition. At night, you WILL convince yourself you've seen things that aren't there. Since I had a wingman to use as a sanity check, it wasn't scary, but I can imagine if I'd been in that state, alone, I'd have been one paranoid SoB. At a certain point, you can tell your body has realized you are NOT going to sleep, and it starts to make up for it to some extent. You're hungrier, and you can just generally feel that you're burning more energy w/o that downtime (part of that was due to the amount of physical exertion I was doing, too).
The final collapse was a BIG shock, tho. I literally walked thru the door, turned and waved, shut the door as they drove off, pivoted on my heel and *darkness*. I can only assume my brain said "You're home, we're done". I slept about 12-14 hours all told between my little carpet nap, the tub and then bed that night. After that, I was good to go.
Now, I'm normally a high-wired guy. I average 6 hours a night of sleep, and commonly get by on 4 for days at a time, and will 'recharge' with a 10 hour weekend night sleep-in on a Saturday or Sunday. This was definitely the limit of what I could do, at least once my brain said "You're home and safe". In a pinch, and in a true emergency, it might be possible to go longer, but I can only imagine that there's a point when you literally can't abuse the system any more.
Oh, for those curious, we didn't even tend to have sodas as camp, so this was done w/o caffeine (other than in candy), any no-doze, or any other kind of stimulant to artificially wire my system. Kool-Aid was on the list, so sugar was a definite go :)
Hope that helps for anyone curious about it :) I'm sure there are much more serious sleep-dep studies out there, but this is the only one I can provide first-hand!