The phenomenon of decrease in performance during the early afternoon hours was observed by researchers as early as the nineteenth century. At first it was thought that the reason was food consumption (thus the term "post-lunch dip”). In the light of the review of research presented in the paper the post-lunch syndrome emerges as a multidimensional mechanism. It has to be stated that the phenomenon is, on the one hand, conditioned by the bi-phase rhythm of sleepiness and, on the other hand, changing balance between the right and the left hemispheres. Still an open-ended question remains if other mechanisms such as fatigue are also responsible for this occurrence.