Evidence check: B12 in the context of sleep

Evidence for a direct link between vitamin B12 and sleep is weak. B12 is involved in homocysteine methylation and melatonin synthesis, but large human trials are lacking. Some observational studies suggest an association between low B12 levels and poorer sleep quality, but causality is not established. Supplementation in non-deficient individuals shows no consistent sleep benefits. MyBody-X tests measure B12, but interpretation is limited. A deficiency should be clinically evaluated before supplementation. Evidence: mechanistic/unclear.