It's known that short-term use has the least consequences but long-term use will produce dependency as it can affect natural melatonin production. People who should use it are those already affected by low melatonin production.

Otherwise, just learning how to sleep and dream naturally can be superior. Lot's of ways to overcome insomnia with techniques for sleep such as PMR (physical muscle relaxation) or TMM (The Military Method).

Always best to go about it naturally if one can is all... supplement only when everything else fails but that can mean removing things like caffeine or other stimulants before sleep.