Overeating in an open-field box increases head-nodding behavior. a A schematic depiction of the new drowsiness model induced by food in an open-field box. One group was not provided with food while the other groups consumed SPAM as a high-fat food or normal food. b The ratio of head nodding behavior showing mice in the no-food, normal food and high-fat food groups during each day (no food; n = 10, normal food; n = 11, high-fat; n = 9). c The number of head nodding was not different among the groups during day-1 and day-2 (one-way ANOVA on ranks, p = 0.637 and p = 0.886, no food; n = 10, normal food; n = 11, high-fat; n = 9). Mice in the high-fat food group showed frequent head nodding during day-3 (one-way ANOVA on ranks, *p < 0.05 and p = 0.292, Dunn’s post hoc test, n.s. indicates ‘not significant’). All error bars represent s.e.m. d Quantification of consumed food between normal food and high-fat food groups (Mann-Whitney U test, p = 0.0088, normal food; n = 11, high-fat; n = 9). All error bars represent s.e.m.