The aims of this study were to document time-related (morning versus afternoon) effects of physical activities (gymnastics) on a set of physiological and psychological variables in school children, including diurnal changes. For the study, 61 boys and 69 girls, 6 to 11 years of age, volunteered. They were considered healthy according to routine clinical criteria. They were synchronized with diurnal activity from around 07:00 to 21:00 and nocturnal rest, time of year being taken into account. Tests were performed at school during 4 weeks of 4.5 days of school at fixed clock hours: 09:00, 11:00, 14:00, and 16:00. Gym time was randomized with regard to week order and season. Four different classes (39 boys and 38 girls) were involved in psychophysiological tests, and two different classes (22 boys and 31 girls) collected saliva samples for morning free cortisol determination. Both t-test and three-factor analysis of variance (ANOVA) were used for statistical analyses. Better performances were obtained in June than in mid-winter with reference to letter cancellation and random number addition tests. As a group phenomenon, morning (09:00 to 10:00) versus afternoon (14:00 to 15:00) gym was not an influential condition with regard to sleep duration, oral temperature, self-rated fatigue and drowsiness, letter cancellation, addition tests, or salivary cortisol. However, gym-time-related differences were observed in classes of younger subjects (e.g., 6-7 years) with regard to self-rated fatigue and the letter cancellation test. Such variability among subgroups suggests that interindividual differences are likely to exist in younger children with regard to manipulation of environmental factors. In addition, gym itself (without gym time consideration) may be an influential factor with regard to diurnal patterns of some variables (e.g., the letter cancellation test).