If you have trouble sleeping, taking a 1 to 3 milligrams of a melatonin supplement in order to help readjust your natural sleep/wake cycle. It should not be needed for long term use.
The science: Melatonin is produced in the brain by the pineal gland around 2 hours before bedtime. Researchers found that taking low doses of melatonin in the evening made it easier to fall asleep and reach deep sleep. The subjects who took the low dose melatonin did not feel groggy from it the next morning (Zhdanova et al., 1995).