Sleepiness is a tricky one. Not like booze where it can be more or less quantified. Being awake for over 24 hours can make you hopelessly dozy, or you could still be wide awake and have plenty of hours left in you before you need a sleep. So purely trying to judge your tiredness by the number of hours you've been awake for is a very poor judgement. Too many other factors will have an effect, which can often be very significant. Strenuous physical activity, which gives a training effect, will mean that your body produces sleep inducing hormones as part of the recovery process. A fitt and strong rider won't have the training effect, so may not feel so sleepy after the same ride.
In short, you have to judge for yourself whether you're in a fit state and err on the side of caution. If you find your mind going astray, your short term memory is fading, you feel sleepy, relatively simple logical thinking becomes more of a challenge then it's probably worth seeing if you can get some sleep before you do anything like driving. Also have a plan of action if you're not fit to drive straight after the event. I often have several plans and schedules for long distance rides, so that if I'm not up to the most ambitious plan, which may be because I think I will get sleepy on a night section, I'll work around it or downgrade my ambitions.
I hope that Simon does continue his excellent night rides. Night riding is a great thing to do, if done well. They are very well designed rides, given that they end near a train station. They seem to do a lot for cycling and it'd be a big shame if they stopped.
I've ridden Audax, I've ridden FNRTTC and I've worked night shifts. If I had to pick the real baddie, I'd pick night shifts, but they all need to be dealt with carefully. It's not a matter of this is good and that is bad. It's a matter of learning your limitations.
