Melatonin is a hormone that your brain produces in response to darkness. It helps with the timing of your circadian rhythms (24-hour internal clock) and with sleep. Being exposed to light at night can block melatonin production. Research suggests that melatonin plays other important roles in the body beyond sleep. However, these effects are not fully understood. Short-term use of melatonin supplements appears to be safe for most people. Many people experience vivid dreams and nightmares while taking melatonin. At higher doses, some people have other side effects including: Feeling confused and disoriented Nausea Vomiting Waking up in the night Dizziness Irritability Bedwetting.