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Jack has a severe caffeine dependency. Pretty much an addiction, and can’t go even a day without his fix before he’s grouchy and head-achey and ready to kill the first person he sees. He becomes pretty much a useless, whining, homicidal mess (so, not far from the usual).
On the contrary, if he has had his caffeine for the day, he’s energetic, almost twitchy, bouncing around his office and has a subconscious refusal to even remotely sit still. With a constant supply, it takes quite a lot to burn him out and get him to sit down let alone sleep; very few are capable of keeping up with it.