When produced naturally in your brain, melatonin helps your body distinguish between day and night; it's what makes you tired when it gets dark, and wakes you when the sun comes up (or when a bright light is turned on in your bedroom).

If something hinders this process, like changing time zones or developing a sleep rhythm disorder, small amounts of synthetic melatonin, sold over-the-counter, may help you readjust to a normal sleep schedule.

Melatonin won't work for every case of insomnia or sleeping problems, though. "Your body has to be ready for sleep for melatonin to work," says David Rapoport, MD, medical director of the New York University Sleep Disorders Clinic. "So for those people who have insomnia because they just don't get sleepy—we call this hyperarousal—melatonin won't do them much good."More nonprescription options

Certain groups of people that lack natural melatonin, blind people or older adults, for example, may benefit from melatonin. But overall, its safety and effectiveness has not been thoroughly studied. Large doses can cause daytime sleepiness and further sleep disruptions, and may interfere with women's ovulation. Read the safety information and talk to your doctor before considering melatonin or its prescription-strength counterpart, Rozerem.