"In the short term, working over 21 hours continuously is equivalent to
being legally drunk. Longer periods of continuous work drastically
reduce cognitive function and increase the chance of catastrophic
error. In both the short- and long-term, reducing sleep hours as little
as one hour nightly can result in a severe decrease in cognitive
ability, sometimes without workers perceiving the decrease."
Definitely worth reading.
Read the article...
Edit: A rather slick optical illusion...