Melatonin is generally considered safe when used short-term and within the recommended dosages. There is no research on the long-term effects of melatonin supplements, particularly in higher doses.
Some experts consider the doses commonly found in melatonin supplements, 3 to 5 milligrams, to be far too high and say that amounts in the range of 0.1 to 0.5 milligrams are more reasonable.
Melatonin side effects may include drowsiness, dizziness, confusion, headache, irritability, vivid dreams, and a temporary reduction in attention and balance. People shouldn't drive or use machinery for several hours after taking melatonin. Melatonin may cause abdominal cramps, nausea, and vomiting, lower blood pressure, and rarely, hallucinations or paranoia.
Melatonin may increase the risk of blood clotting, so it should not be used by people using warfarin (Coumadin), other medications that influence blood clotting, or by people with clotting disorders.Melatonin influences the production of other hormones. It could theoretically interfere with normal sexual development, so it shouldn't be used by children unless they are under the supervision of a healthcare provider. For the same reason, it shouldn't be used by women who are trying to conceive or by pregnant or nursing women. Increased male breast size
and reduced sperm count have also been reported. Melatonin may also affect insulin levels.
Melatonin can influence immune function and it's not known how it affects people with autoimmune conditions such as multiple sclerosis, psoriasis, Crohn's disease, rheumatoid arthritis, lupus, and type 1 diabetes.
Melatonin supplements may worsen the symptoms in people with depression, so people with depression should only use melatonin under the supervision of a health care provider. Melatonin is broken down by the liver, so people with liver disease should avoid melatonin.
I think I'd rather adjust my training cycles.