This study evaluates the effects of milk intake for 20 days at breakfast on diurnal type (chronotype), sleep habits and soccer performance in Japanese university male athletes attending a university soccer club. Seventy three athletes were asked to take 200 ml of cows’ milk at breakfast for 21 consecutive days during November and December, 2014. Twenty athletes attending the same soccer club did not drink cows’ milk for the same period of time and acted a control group. An integrated questionnaire was administered twice, before the intervention period and 1 month after it to all 93 participants. The questionnaire included questions on sleep habits and diurnal type. On the 10th day and 21st days of the intervention period, a questionnaire on performance/skill was administered to all participants. The group which drank cows’ milk showed higher frequency of improvement of soccer performance than did the control group did (performance—where higher values indicate less skill: milk drinking group=29.92, control group=31.9 on day 10; milk drinking group=28.21, control group=31.9 on day 21), and also judged that their soccer performance had improved more after 21 days than 10 days of the intervention. Those participants who changed diurnal type to becoming more morning-typed were more likely to judge that their soccer performance had improved than did those who showed no change in diurnal type.