Background: Sleep quality and anxiety regulation are essential for recovery, cognitive function, and readiness in military cadets. High-intensity interval training (HIIT) and melatonin may influence sleep and psychological responses, but their combined effects remain unclear.

Purpose: This study examined the isolated and combined effects of four weeks of HIIT and melatonin supplementation on sleep quality and anxiety in military cadets.

Method: In this quasi-experimental pretest-posttest study, 31 male military cadets were allocated to Melatonin (n = 10), HIIT (n = 10), and HIIT + Melatonin (n = 11) groups. Training groups performed supervised running-based HIIT three times weekly for four weeks. Supplementation groups consumed 5 mg melatonin nightly at 21:00. Sleep quality and anxiety were assessed using the Pittsburgh Sleep Quality Index (PSQI) and Beck Anxiety Inventory (BAI). Paired-sample t-tests and ANCOVA were used for statistical analysis.

Results: PSQI scores improved significantly in all groups (P < 0.01), with the largest within-group improvement observed in the HIIT + Melatonin group (P = 0.001, Cohen's d = 1.33). Anxiety scores also decreased significantly in all groups (P < 0.05), with the largest reduction observed in the combined group (P = 0.002, Cohen's d = 1.25). ANCOVA demonstrated significant between-group differences in adjusted post-test PSQI and BAI scores after controlling for baseline values.

Conclusions/Implications: HIIT and melatonin improved sleep quality and reduced anxiety in military cadets. The combined intervention demonstrated the largest within-group improvement and may represent a useful recovery-oriented strategy for military training programs.