Melatonin is pretty good stuff indeed, and it kills cancer cells to boot. It's a hormone your body produces to trigger sleep, in case it wasn't mentioned yet.

Just a little something interesting, smoking marijuana by a first time toker (or just one with a really low tolerance) causes an average 5,000% increase in your natural melatonin levels, hence why you get the "burnout" at the end and many get sleepy during the high. The reason you don't get sleepy during, at least not as bad a the burnout, is because of the seratonin increase that comes with the stone.