pilotbear,
you have a good point about the effect of altitude.
The following is an extract from a bmj article on the effect of length of "wakefulness" on performance of of simple mental and co-ordination tasks.
The fact that it is compared to the effects of alcohol, and has been done at "only" sea level leads one to wonder how safe the current duty limits are in the real world. Please note the legal driving limit is the comparison point, not the 20mg per dl.
http://oem.bmjjournals.com/cgi/content/full/57/10/649
Quote (from way down near the end)
"The overall implications of the results of this study are clear. They show that the effects on performance of moderate periods of being awake cannot be discounted. Sleep is needed after the end of a day if adverse effects on performance are to be avoided. Most importantly, this study has allowed interpretation of these effects on performance in terms of an accepted standard for safety. With a legal limit for alcohol use when driving as a standard, the results show that after 17-19 hours of wakefulness, subjects' performance on many tests had dropped to that found at the legal limits for safe driving. Many people remain awake for periods of 16 hours or more for reasons of work, family, or social life. These results suggest that after this duration of wakefulness fatigue reaches a level that can compromise safe performance.
The results also imply that many countries which set allowable BACs at the point that compromises safe performance should consider developing similar standards for fatigue to ensure that people who have had 18 hours or longer without sleep are kept from at risk behaviours such as driving, piloting aircraft, or operating machinery. "