This randomized, double-blind, placebo-controlled trial will compare magnesium glycinate, magnesium L-threonate, and placebo in UCLA varsity athletes. Participants will complete a baseline monitoring period followed by 4 weeks of blinded nightly supplementation. WHOOP or study-approved wearable data will be used to evaluate sleep efficiency, total sleep time, sleep consistency, heart rate variability, resting heart rate, and recovery metrics. Baseline and final testing will assess selected reaction and physical performance outcomes. The primary outcome is change in WHOOP-derived sleep efficiency from baseline week to final treatment week.

Magnesium is involved in neuromuscular signaling, cellular energy metabolism, autonomic regulation, and sleep-related physiology. Competitive athletes often experience sleep restriction, training stress, travel, soreness, and recovery demands. Magnesium glycinate and magnesium L-threonate are commonly used athlete-facing magnesium formulations, but they are not interchangeable. Glycinate is commonly positioned as a well-tolerated sleep-oriented formulation, whereas L-threonate is of interest because of prior signals related to sleep, cognition, reaction performance, and central nervous system magnesium biology. This single-site UCLA study will enroll adult varsity athletes aged 18 to 35 years. Participants will complete screening, informed consent, baseline assessments, wearable monitoring, a baseline monitoring period, randomization in a 1:1:1 ratio, 4 weeks of blinded nightly capsules, brief daily REDCap morning surveys, weekly adherence and safety check-ins, and final performance-adjacent testing. The primary outcome is change in average WHOOP-derived sleep efficiency from baseline week to final treatment week. Prespecified primary contrasts will compare magnesium glycinate versus placebo and magnesium L-threonate versus placebo. Total sleep time, sleep consistency, resting heart rate, heart rate variability, WHOOP Recovery Score, reaction-time performance, grip strength, countermovement jump height, adherence, tolerability, and adverse events will be analyzed as secondary or exploratory outcomes according to the final statistical analysis plan. The magnesium glycinate versus magnesium L-threonate contrast will be treated as exploratory unless the final statistical analysis plan preserves alpha for that comparison.

Inclusion Criteria:

• Age 18 to 35 years.
• Current UCLA varsity athlete.
• Actively training or competing during the study period.
• Willing to wear WHOOP or a study-approved wearable device continuously during baseline and treatment periods if wearable data are used.
• Willing to take assigned study capsules nightly for 28 days.
• Willing to complete brief daily REDCap surveys and weekly adherence/safety check-ins.
• Able to provide informed consent and comply with study procedures.

Exclusion Criteria:

• Current magnesium supplementation without completion of an appropriate washout before baseline.
• Current investigational drug or investigational supplement use.
• Current use of prescription or over-the-counter sleep medications unless reviewed and permitted by the study clinician.
• Diagnosed sleep disorder that, in the investigator's judgment, would confound outcomes or increase risk.
• Significant kidney disease or another medical condition that may increase risk with magnesium supplementation.
• Known intolerance or allergy to magnesium glycinate, magnesium L-threonate, placebo, or inactive study ingredients.
• Use of medications with clinically relevant magnesium interactions unless reviewed and permitted by the study clinician.
• Any other condition that, in the investigator's judgment, would make participation unsafe, compromise voluntary consent, or prevent valid outcome assessment.

No individual participant data sharing is planned because the study involves a relatively small cohort of collegiate athletes and individual-level sleep, recovery, performance, and sport-related data may create re-identification risk. Aggregate results may be disseminated through presentations, manuscripts, and ClinicalTrials.gov summaries as applicable.

Participants, study personnel involved in assessment, investigators, and outcome assessors will remain blinded to treatment assignment. The primary analysis will also be performed with treatment assignment masked when feasible. Emergency unblinding may occur only when needed for participant safety or regulatory requirements.