Tryptophan can be metabolized via 5-hydroxytryptamine=serotonin to melatonin by a series of 4 enzymes in pineal body. Lack of serotonin in body fluid in the brain during daytime can lead to several psychiatric disorders, while shortage of plasma-melatonin at night can be related to sleep disorders. The Morningness–Eveningness (M–E) questionnaire and the original questionnaire including questions on sleep habits, mental symptoms, and contents of meals were administered to 1055 infants aged 0–6 yrs, 751 students attending an elementary school, and 473 students attending junior high school in Kochi City (33°N). The index of tryptophan taken at breakfast (Trp-Index) was calculated as tryptophan amount per one meal based on the tryptophan included in each 100 g of the foods and a standard amount of food per one meal. A significant positive-correlation between M–E scores and Trp-Index was not shown by relatively older students, aged 9–15 yrs (Pearson's test, r =0.044–0.123, p =0.071–0.505), whereas a significant positive correlation was shown by infants and young elementary school students aged 0–8 yrs ( r =0.180, 0.258, p <0.001). The more frequently the infants had difficulty falling asleep at bedtime and waking up in the morning, the less the Trp-Indices taken at breakfast were (Kruskall–Wallis-test, p =0.027 for difficulty falling asleep; p =0.008 for difficulty waking up). The more frequently infants became angry even by a little trigger, or depressed, the lower (more evening-typed) the M–E scores were (Kruskal–Wallis test: p ≤0.001). Tryptophan ingested at breakfast is very important for children to keep a morning-type diurnal rhythm, high quality of sleep, and indirectly good mental health, presumably, through the metabolism of tryptophan to serotonin in daytime and further to melatonin at night.