Is melatonin safe to take on a regular basis for sleep?

Probably safe. Melatonin is secreted by the pineal gland and acts to regulate the body's sleep/wake cycle. Its use as a supplement has been proposed for multiple reasons; improved sleep, chronic fatique and autism are just a few. No serious toxic effects are reported, but caution is advised when used with other sedative meds or when anticoagulents or dibetic meds are involved. Talk to your doc to learn more.
Seems to Be. Current studies demonstrate effectiveness with safety.