Paired studies were conducted in 18 healthy volunteers (9 men, 9 women) to investigate whether differences in mood and daytime sleepiness induced by high-fat-low-carbohydrate (CHO) and low-fat-high-CHO morning meals were associated with specific hormonal responses. Plasma insulin concentrations were significantly higher after low-fat-high-CHO meals, and cholecystokinin (CCK) concentrations were significantly higher after high-fat-low-CHO meals. Subjects tended to feel more sleepy and less awake 2-3 h after the high-fat-low-CHO meal, and ratings of fatigue were significantly greater 3 h after the high-fat-low-CHO meal than after the low-fat-high-CHO meal. The results of the present study are consistent with the hypothesis that there is an association between the lassitude experienced after a meal and the release of CCK.