https://www.vice.com/en_us/article/pkgakv/why-does-thinking-hard-make-me-so-tired

In 1998, researchers baked chocolate chip cookies in order to fill a laboratory with the aroma of freshly made treats. Then, they brought people into the lab, and sat them in front a stack of aforementioned cookies, along with some chocolate candies, and a bowl of red and white radishes. 

People were put in either the radish group or the sweets group, and told to eat only the food that had been assigned to them, for the purposes of a “taste perception study.” Then an experimenter left the room, and watched the participants behind a one-way mirror.