Are you saying that you cant tell the difference between working normally and working a significantly below normally, or that even when you're seriously impaired you feel fine (eg after being awake for 30 hours)?

I cannot, or at least until it gets to really extreme levels, and not reliably in any case.

My sleepiness is not terribly correlated with my levels sleep deprivation - this could be taken to ridiculous levels back when I tried Provigil, as I didn't feel sleepy or tired or impaired in any way, after weeks of sleeping just a couple of hours a day, even though my performance suffered massively (as verified retrospectively by svn logs and such), and got back to high levels as soon as I slept it over.

Normally it's not as extreme, but significant drop in performance happens many hours before I feel sleepy or tired, and can usually be fixed by a short 1-2h nap. It's even worse, as I don't know how sleepy I am - often I know I need some sleep, but my track record of predicting if it will be 1-2h nap or a full 8h sleep is hardly better than random. People with more regular 24h activity cycles probably don't have this problem as much.