This study aims to compare moderate-intensity swimming exercise in the morning and at night towards the increment of irisin levels in female mice (Mus musculus). This research is a real experiment with the randomized control group post-test-only design. A total of 24 female mice (Mus musculus), aged 3 months old, and weighing 20-40 grams (Lee’s Index Value > 0.3) were randomized into three groups, namely G1 (n = 8, control without intervention), G2 (n = 8, moderate-intensity swimming exercise in the morning), and G3 (n = 8, moderate-intensity swimming exercise at night). Swimming exercises were carried out 3x/w for 4 weeks with an intensity of 6% of body weight and the duration was 70% of maximum swimming time. Irisin levels were measured using the ELISA method. The data analysis techniques used were one-way ANOVA test and Tukey's HSD post-hoc test. The results that were obtained from this experimental study were mean levels of irisin in G1 (1.86 ± 0.06 ng/mL), G2 (2.66 ± 0.12 ng/mL), G3 (3.43 ± 0.35 ng/mL), and (P ≤ 0.001). The results of Tukey's HSD post-hoc test showed that there was a significant difference in the mean post-training irisin levels between G2 and G1 (P ≤ 0.05), G3 and G1 (P ≤ 0.001), and G3 and G2 (P ≤ 0.05). Based on the results of this study, it was concluded that moderate-intensity swimming exercises in the morning and at night were able to increase irisin levels. However, moderate-intensity swimming exercise at night showed a higher effectiveness in increasing irisin levels of female mice (Mus musculus). Keywords: Obesity, Irisin levels, Morning exercise, Night exercise

This study aims to analyze the effect of moderate-intensity swimming exercise which performed in the morning and at night on reducing body weight and Lee's obesity index in female mice (Mus musculus). This study used the true experiment method with the randomized control group post-test-only design. A total of 30 female mice (Mus musculus) at 3 months of age, and weighing 30 -40 grams (Lee's Obesity Index Value > 0.3) were divided randomly into three groups of 10, namely CTRL (n = 10, control without intervention), MISEM ( n = 10, moderate-intensity swimming exercise in the morning), and MISEN (n = 10, moderate-intensity swimming exercise at night). Body weight and Lee's obesity index was measured twice, namely preexercise and 4 weeks post-exercise. Then, data was analyzed using the Paired Samples T-Test with the Statistical Package for the Social Sciences SPSS version 21 software.The results show that the average body weight on CTRL between pre-exercise vs. post-exercise (33.60±2.17 vs. 39.20±3.82 gr (p-values=0.001)), )), and MISEN (33.40±3.47 vs. 26.00±2.26 gr (p-values=0.000)). Lee's average obesity index on CTRL between pre-exercise vs. post-exercise (0.39±0.02 vs. 0.44±0.02 gr/mm (p-values=0.000)), MISEM (0.40±0.02 vs. 0.42±0.01 gr/mm (p-values=0.000)), MISEN (0.41± 0.01 vs. 0.37±0.02 gr/mm (p-values=0.002)). Based on the results of the study, it was concluded that moderateintensity swimming exercise in the night was more effective in reducing body weight and Lee's obesity index than moderateintensity swimming exercise in the morning and control without intervention.