Effects of exercise intensity on circadian rhythm and glucose regulation in adults with obesity: a randomized crossover trial

[Purpose] Circadian rhythm disruption is associated with metabolic dysfunction and obesity. While exercise can serve as a non-photic zeitgeber, the optimal intensity for evening exercise sessions in adults with obesity remains unclear. This pilot crossover trial compared the acute effects of evening high-intensity interval training (HIIT) versus moderate-intensity interval training (MIIT) on circadian core body temperature rhythm, 24-hour glucose regulation, and sleep quality in adults with obesity.[Methods] Nine adult males with obesity (BMI ≥ 25 kg/m², Asia-Pacific criterion) completed a randomised crossover trial of HIIT (~30 min at 90% heart rate reserve [HRR]) and MIIT (~50 min at 60% HRR) on a cycle ergometer between 19:00 and 21:00, separated by a 7–14-day washout. Twenty-four-hour core body temperature (ingestible telemetric capsule), interstitial glucose (FreeStyle Libre 1), and wrist actigraphy were recorded for 24 h before and 24 h after each exercise session under standardised dietary intake. The primary outcome was the change in circadian temperature amplitude (Δ amplitude) derived from a 24-hour fixed-period cosine fit with non-negative amplitude bound; mesor and acrophase were co-primary. Effect sizes (Cohen’s d_z) are reported throughout.[Results] Δ amplitude differed in direction between conditions (MIIT −0.01 ± 0.19 °C; HIIT +0.12 ± 0.24 °C) but did not reach conventional statistical significance (paired <i>t</i>(8) = −2.19, <i>p</i> = 0.060, d_z = 0.73). Mesor (Δ contrast <i>p</i> = 0.49, d_z = 0.24) and circular acrophase (Δ contrast <i>p</i> = 0.67, d_z = 0.15) did not differ between conditions, with both protocols producing approximately one-hour phase advances. Twenty-four-hour mean glucose post-exercise trended lower following MIIT (95.9 ± 6.1 mg/dL) than HIIT (98.7 ± 5.5 mg/dL; <i>p</i> = 0.081, d_z = −0.72); coefficient of variation, time in range, and mean amplitude of glycaemic excursions did not differ. Post-exercise sleep efficiency, total sleep time, and wake after sleep onset did not differ between conditions (all <i>p</i> > 0.50; |d_z| ≤ 0.24).[Conclusion] In this single-session pilot crossover trial in nine adult males with obesity, evening HIIT and MIIT produced acute changes of similar magnitude in mesor and phase, with a borderline non-significant between-condition difference in amplitude change. Twenty-four-hour mean glucose showed a comparable non-significant trend toward lower values following MIIT, whereas glucose variability and sleep markers did not differ between conditions. Larger work-matched trials are required to determine whether evening exercise intensity produces clinically meaningful differences in circadian or metabolic outcomes in adults with obesity.