The sense of sleepiness and fatigue that occurs at around 2 p.m. is known as the “post-lunch dip (PLD).” It causes a transient decline in brain function, including cognitive function, attentiveness, and arousal level. Various research hypotheses have been proposed for the mechanism of occurrence of PLD, including explanations involving blood sugar spikes or the inhibition of neuropeptides. However, the evidence for these hypotheses is poorly constructed, and none of them is widely recognized as an explanation for the mechanism. The establishment of quantitative evaluation indicators for the decline in brain function caused by PLD is essential to clarify the mechanism of occurrence of PLD. In this study, a demonstration experiment was conducted focusing on P300 and contingent negative variation (CNV), which are types of event-related potentials (ERP), as evaluation indicators of PLD. The subjects were 14 healthy young people, and the meal load used was two slices of white bread and 285 mL of water. In the experiment, measurements were taken four times in total (preprandial, immediately postprandial, 40 min postprandial, and 80 min postprandial). The Stanford sleepiness scale (SSS) and a subjective questionnaire about fatigue using a visual analog scale (VAS) were administered before each measurement. The results confirmed that, at 40 min postprandial, when a significant increase in SSS was observed, there was a reduction in the area of early CNV and late CNV and a prolongation of P300 latency (p<0.05). An evaluation using late CNV also confirmed a reduction in area immediately postprandial that could not be confirmed in the SSS.

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