A university professor researching sleep disorders occasionally taught class after spending whole nights working in a laboratory  She found lecturing after such nights difficult: she reported that she felt worn out and humorless, and she had difficulty concentrating and finding the appropriate words  After several weeks of lectures, she asked her students to guess which lectures had been given after nights without sleep
Interestingly, very few students were able to correctly identify them
University students in a lecture audience tend to be astute observers of human behavior.