Human studies have shown that melatonin is not toxic in doses usually used for sleep, 0.5–5 mg. In addition, melatonin was not found be lethal in animal studies using hundreds of times the amount of normal human doses. Although melatonin has been found to be nontoxic, there is an increased risk of side effects with higher doses, including drowsiness, dizziness, abdominal cramps, hangover effects, irritability, and impaired balance. High doses of melatonin can worsen depression. Long term studies of melatonin have not been completed. However, many people with circadian rhythm disorders (many of them blind) have taken melatonin for many years.