That's the time that the subjects stopped consuming caffeine as part of their usual diet, not the time that the pill (either 200 mg caffeine or a placebo) was a

That's the time that the subjects stopped consuming caffeine as part of their usual diet, not the time that the pill (either 200 mg caffeine or a placebo) was administered. The subjects were required to maintain their usual caffeine intake (to prevent withdrawal), but had to stop by noon to prevent that caffeine from interfering with the experiment.