Correlation scatter graph for subjective sleepiness and amygdala activity. In fear-neutral contrasts, amygdala activity is significantly positively correlated with subjective sleepiness during a face-viewing task (r = 0.71, p < 0.001). Amygdala activity was obtained by averaging clusters in the bilateral amygdala that showed a significant positive correlation in correlation analysis (p < 0.001, uncorrected). Data from the SC and SD sessions are shown in different colors in the graph. ●, SC; □, SD. Abbreviations: SC = sleep control condition; SD = sleep debt condition; Degrees of freedom (df) = 20.