For athletes, it is important to acquire lean body mass (LBM) involving the skeletal muscle mass during their growth periods; however, the influence of chronotype on LBM gain remains unclear. We therefore aimed to investigate whether chronotype, sleep-wake cycle on weekdays (SWC-W), and their interaction contribute to LBM gain among adolescent male athletes in a 4-month intervention study. The participants were 45 male high-school baseball players. The intervention, including exercise menu (running and muscle strength training) and nutritional education, was conducted during a 4-month period of season-off training. The chronotype, body composition, lifestyle, and dietary intake were investigated before intervention (baseline) and after 4 months. Among the participants [Morningness (= 14), Eveningness (= 15), Intermediate (= 16); ME score based on the Morningness/Eveningness Scale for Children (MES-C)], the midpoint of sleep on weekdays (MSW) was calculated in the "Morningness" and "Eveningness" participants, respectively. They were divided into 4 groups based on a match/mismatch with the chronotype: Type M-match (= 8), Type M-mismatch (= 6), Type E-match (= 7), and Type E-mismatch (= 8) groups. The data were compared among the 4 groups. Moreover, multiple regression analysis was conducted using an increase (kg) LBM gain as a response variable. When comparing the data between the "Morningness" and "Eveningness" participants, there were no differences in nutrient intake, the duration of training, or each parameter of body composition (per body weight) at baseline or after 4 months. There were also no differences in the rates of change in the body weight or each parameter of body composition. In groups in which the chronotype was consistent with the SWC-W (the Type M-match and Type E-match groups), the LBM gain were slightly greater than in the Type M-mismatch and Type E-mismatch groups (Type M-match: 3.5 ± 2.0 kg, Type M-mismatch: 1.6 ± 1.7 kg, Type E-match: 3.4 ± 2.2 kg, and Type E-mismatch: 1.2 ± 1.8 kg,= .057). Multiple regression analysis revealed that an extent of the LBM gain was associated with a match between the chronotype and SWC-W (ß = 0.37,= .030), independent of a long duration of training (ß = 0.52,= .004). The results suggested that training-related LBM gain is associated with interactions between the chronotype and SWC-W in adolescent male athletes.LBM: Lean body mass; SWC-W: Sleep-wake cycle on weekdays; ME score: Morningness-eveningness score; MES-C: Morningness/Eveningness Scale for Children; MSW: Midpoint of sleep on weekdays; MSF: Midpoint of sleep on free days; MSFsc: Midpoint of sleep on free days corrected for sleep debt accumulated through weekdays. n n n n n n n p p p Abbreviations:
