 Why does Simon look for aspirin as soon as he gets to his office?
He sat down at his desk, passed his tongue over his teeth in distaste, groaned, fumbled in a drawer for the aspirin bottle. He looked over at Betty and said, almost as though reciting, "What I need is a vacation." "Providence," Simon told her whilst fiddling with the aspirin bottle, "will provide." Something where you could deduce that actually the ruby ring had gone down the drain and was caught in the elbow. Betty bounced up with Olympic agility and had the door swinging wide before the knocking was quite completed. The client fussed himself with Betty's assistance into the seat, bug-eyed Simon, said finally, "You know my name, that's pretty good. When Simon continued to say nothing she ventured, "Time travel is impossible." "Then you'll realize that there are a dozen explanations of the paradoxes of time travel. It's my contention that within a century or so man will have solved the problems of immortality and eternal youth, and it's also my suspicion that he will eventually be able to travel in time. "The usual explanation, Betty, is that they can't afford to allow the space-time continuum track to be altered. If, say, a time traveler returned to a period of twenty-five years ago and shot Hitler, then all subsequent history would be changed. Mr. Oyster returned his glasses to their perch, bug-eyed Simon, but then nodded. Simon said, "You want to hire me to find a time traveler and in some manner or other—any manner will do—exhort from him the secret of eternal life and youth, which you figure the future will have discovered. Now she said, plaintively, "But where are you going to find one of these characters—especially if they're interested in keeping hid?" People would figure they had D.T.'s." If you could go back a few thousand years, the things you would wish to see would be a Roman Triumph, perhaps the Rites of Dionysus, or one of Alexander's orgies. "No ma'am, you'd have to stick to some great event, both for the sake of actual interest and for protection against being unmasked." You can take the plane to Munich, spend a week there and—" Simon was shaking his head. "A hundred dollars a day plus expenses," Mr. Oyster said quietly.
 He has a hangover from attending a festival.
 He is experiencing caffeine withdrawal and did not have time to stop for coffee.  We never learn the cause of the headache, we only know that it is severe.  He has a hangover from attending a festival.  He was out drinking with some friends the night before, and has a hangover.