Is CLOCK useful for fatigue?

The utility of CLOCK genotyping for fatigue is not supported by strong evidence. CLOCK (Circadian Locomotor Output Cycles Kaput) is a core circadian gene that influences sleep-wake cycles and energy metabolism. Circadian disruption is linked to fatigue, and some studies report associations between CLOCK variants and chronic fatigue symptoms. However, the evidence is primarily mechanistic or observational; no clinical trials demonstrate that using CLOCK genotype information improves fatigue outcomes. Effect sizes are modest and confounded by lifestyle factors. Consumer genetic tests may claim relevance, but current guidelines do not recommend CLOCK testing for fatigue management. Practical steps such as consistent sleep schedules, light exposure, exercise, and stress reduction are more impactful. Therefore, CLOCK is not considered useful for fatigue in a clinical or actionable sense. Evidence level: mechanistic.