When time matters: Circadian rhythm outweighs menstrual cycle in strength, but not in motivation

OBJECTIVES: We investigated the independent and combined effects of time of day (ToD) and menstrual cycle (MC) on strength performance and motivation. An additional aim was to examine whether chronotype moderated these relationships. DESIGN: Observational, within-subject, repeated-measures study. METHODS: Twenty-seven naturally menstruating, physically active females completed six testing sessions at two ToDs (morning: 07:30-09:00 h; afternoon: 16:30-18:30 h) and across three MC phases (early follicular, ovulation, mid-luteal). Strength was assessed using handgrip dynamometry, countermovement jump (CMJ), and isokinetic knee flexor and extensor dynamometry. Motivation was measured using a 7-point Likert scale. RESULTS: Strength was significantly higher in the afternoon: handgrip (+0.7 kg, p = 0.026), CMJ height (+0.016 m, p < 0.001), CMJ power (+2.5 W/kg, p < 0.001), dominant knee extensor (+5.86 Nm, p = 0.007), and non-dominant knee extensor (+4.17 Nm, p = 0.020). Motivation peaked during estimated ovulation, being significantly higher than in the early follicular (+0.89 points, p = 0.006) and mid-luteal (+0.65 points, p = 0.036) phases. A significant interaction between ToD and MC was observed only for non-dominant knee extensor strength, with higher values in the afternoon compared to the morning during both estimated ovulation (+7.12 Nm, p = 0.038) and the mid-luteal phase (+5.69 Nm, p = 0.012). Including chronotype revealed that motivation varied significantly across chronotypes (p = 0.032). CONCLUSIONS: Strength was more consistently influenced by ToD, whereas MC was primarily associated with motivation. These findings highlight the importance of considering circadian rhythm in training and testing for females.