20 years ago, I read somewhere in one of my college psyche or bio (was double-major) textbook that a study was done using milk spiked with caffeine and decaf coffee.

Two groups of students cramming for exams were studied. One group was given milk with same amount of caffeine as regular cups of coffee, and other group was given decaf. Neither groups were told about the caffeine content and the decaf group thought they had regular coffee while caffinated milk group thought they had plain milk.

While decaf folks thinking that they had caffeine and being able to stay up and study wasn't surprising, the study found that a large portion of the milk drinkers reported not being able to stay awake.