That reminds me of Rule 4 in the "Homicide Lexicon", in David Simon's fantastic non-fiction book, "Homicide", of the Baltimore homicide squad:

"An innocent man left alone in an interrogation room will remain fully awake, rubbing his eyes, staring at the cubicle walls and scratching himself in dark, forbidden places. A guilty man left alone in an interrogation room goes to sleep."

http://en.wikipedia.org/wiki/Homicide:_A_Year_on_the_Killing...