Discworld fic. Vetinari/ Vimes, what else. And probably done before by someone else, because it's so obvious when you think about it.
ETA: Why does rich text format have to be called rich text format? Now I have a Word file called vetinari rolling on the floor. Laughing, presumably.
Vimes came at his summons but clearly under protest. It was clear because the first thing he did was protest: "It's bloody 2 a.m. Do you never go to bed?"
"Well, if you'd-- Wait. What? No? You don't sleep?"
"Of course I sleep. I'm not a vampire."
Vimes' suddenly blank expression said it all.
"Yes, I'm quite sure, thank you, Vimes. For various reasons of my own I find it preferable to sleep at my desk. Now, about this report of yours---"
"I beg your pardon?"
"What reason can you possibly have for not living like other people? Does it add to your alertness? Your acuity? Or just your legend? 'The Unsleeping Master of Ankh-Morpork'? 'He Plots While His City Slumbers'? Something like that, is it, sir?"
"My, my. You do have your dander up, Commander. Is this mutiny?"
"No." The syllable dropped between them like a challenger's gauntlet.
Vetinari sat back in his chair and sighed. It was possible to push people too far. Circumstances and Vetinari together seemed to have done that now to Vimes.
"Do you remember," he said, "once, a long time ago, I gave you a piece of advice? Don't bother answering: I'm sure you forgot it as soon as you were out the door. But you asked me if I really believed what I said, about the endless petty evil of the human race. And when I said yes of course, there's nothing else one reasonably can believe, you asked why I still got out of bed in the morning?"
"Yes," Vimes said slowly. "And you didn't answer me."
"Naturally not. But that's why I find it preferable to sleep at my desk.