I was thinking about the fleche after I posted. I don't remember being particularly sleepy on that ride. Maybe the problem on other 400k+ rides is that they typically start at 4 in the morning so I don't get good sleep the night before.
This is an old thread now, but the recent answers remind of another factor.
When I did the 48-hour race and the 24-hour race, start time was at 5:00 or 6:00 in the evening. "No problem", thought I, "I'll just sleep all afternoon, wake up all refreshed and go ride umpteen hours without sleep! Yeh!" But then when I laid down to sleep all afternoon, I wasn't the least little bit sleepy, so that plan got ruined in a hurry.
The moral is, probably most people can go 24 hours without sleeping under the right conditions, but if you've already been awake 8 or 12 hours when that 24 hours starts, that's different! And if you didn't get quite enough sleep the night before anyway, that's even worse.
Keeping adequately refuelled and rehydrated throughout the period will help get through without sleep for 24 hours. The brain operates on glucose just as muscles do, and it needs as much nourishment. I used to mix my own "jungle juice" powder mix including Lite Salt, but I've also used Heed. Drinking the calories helps on two fronts -- the refuelling, obviously, and the rehydration.
I did start to develop mouth ulcers drinking the malto and other sugars, and I think I still prefer eating solid foods, mainly because my intensity is not very high, but measured out across an event. Over 24 hours or long, there is a need to know that ordinary food can be available along the route. 24-hour McDs or other fast-food outlets have helped there.
Coca-Cola (and for me, only Coke) also can keep me going long after I should have stopped. It likely is the caffeine, but the sugar hit helps a bit.
The longest ride I've done with minimal sleep was a 1,000 km brevet. I slept for maybe 30 minutes at one of the controles. We went over 3 major climbs (Coquihalla and Pennask in BC, Stevens Pass in WA). It rained on the descent of Stevens Pass, and also in the last few hours, but I felt OK when I finished in 45 hours. I did start hallucinating on the drive home an hour later, though! So for me, at least, I figure my maximum is 45 hours non-stop.
So, I think the limiting factor for a ride without support would be the point at which you start hallucinating. I would think you'd be entering the risk zone after 40 hours straight. And to calculate how far that would be, I generally use my own rule of thumb of a 25 kmh average speed plus an hour for every 1,000 meters of climbing (15 mph + 1 hour for every 3,000 feet).
I guess it depends on what you call a hallucination. I have had visual hallucinations at night in the first 24 hours. I've gotten used to them. Our brains work pretty hard to extract meaning from what we see, and when we're tired it doesn't do that great of a job
One problem with the sleep is that, for me, it's kind of unpredictable how it's going to work out. I've also found that using caffeine, after a while, it'll keep you awake, but not alert, you get all zombified instead. So maybe you can finish a ride, but whether you're doing it safely is a different issue.
Oh- an experience comes to mind. I was riding the second day of a 600k, middle of the afternoon, it was warm, I had a tailwind, it was flat. So easy riding. I was riding ahead of the group by maybe a 100 yards. I was watching for the little town of Fairy, where an FM road turned off to the right. I was watching my odometer, and when I got a couple of miles past that, I hadn't seen anything, and looked back and didn't see the group either. So I backtracked and discovered that I ridden right past Fairy, right past the FM road, without seeing or realizing any of it. I caught back up with the group, and they said they had all hollered and whistled and I just kept right on riding. So whatever mental state I was, I was able to keep my bike on the road and upright, but it's hard to say that I was riding safely at that point. I might just be very lucky a semi didn't come along about then.
Bob would fall back on every climb, I would soft pedal until he caught up, ride together, rinse, repeat.
But on Piney Grove - Wilbon Rd, I was daydreaming and missed the turn onto Burt Rd.
Bob later said he yelled, waved, blew his air horn, but was not going to follow me the wrong way.
I realized my mistake when I got to Duncan Cook Rd -- about 3/4 of a mile, I think.
I rode back to Burt Rd, and then figured that I would catch Bob in 8 or 10 miles.
TomD "caught" me when I stopped at the Apex Fire Station on New Hill - Olive Chpl Rd (?) to get some cold water, and we rode in the rest of the way together.
Tom and I saw Bob when we turned off of NC-55 onto Morrisville Pkwy.
That was a relief because I was worried that he was somewhere back near NC-42, in trouble.
But kept thinking that if he were in trouble, he would call.
No call came, so I kept going.
It had taken about 24-miles to make up for that missed turn.
With cooler temps and the ibuprofen doing its job ...
Seemed like Bob was "riding like the wind."
Full story here: http://irregularveloadventures.blogs...ox-600-km.html
Bob's version of the story here: http://bikeroundnc.blogspot.com/2013...unds-600k.html
I don't think I was necessarily sleep deprived when I missed the turn.
My recollection is that I was thinking about ... who knows what!
And, for the final bit of humor regarding me missing the turn --
I make the maps for Alan's brevets.
I had to redraw the map because of the switch to RWGPS.
It is the same route every year, and this was the third time I have completed the route.
One would think I would know the route.