Here's a link:

http://www.cures-not-wars.org/melajuana.html

This is the first paragraph:

Of all the known ways to stimulate melatonin production, none is more dramatic than smoking marijuana. Marijuana stimulates production of a prostaglandin called PGE2, which may relate to its ability to stimulate melatonin production. Italian researchers discovered that when eight men smoked a cigarette containing the active ingredient in marijuana, THC (tetrahydrocannabinol), they had dramatically higher melatonin levels twenty minutes later. After two hours, their melatonin levels were 4,000 percent higher than at baseline!(11)