Gelatin and glycine have bounced around the blogosphere for quite some time. Coming from a nutrition-centric place: you say gelatin, I think tryptophan (or lack thereof) and glycine. Others think:
Jane Plain discusses positive mental effects of gelatin and pimps Pro-Stat (good source of glycine). Chris Masterjohn discusses glycine and in typical WAP fashion seems to favor bone broth. Knox gelatin didn’t help Michael Allen Smith sleep better, and he apparently tracks sleep quality quite well. However, Sondra Rose thinks it improves sleep harmony, and gelatin simply blows Dana Carpender’s mind.
Tryptophan-rich proteins like those found in whey and egg whites will elevate blood levels of tryptophan relative to other large neutral amino acids (Trp:LNAA ratio), leading to higher brain uptake and subsequent serotonin synthesis. Tryptophan-poor proteins like gelatin do the opposite, and impair memory. But the high glycine content in gelatin improves sleep quality.* Glycine powder might be able to get around this, it’s dirt cheap and it seems to have the opposite effect on brain serotonin, albeit at a much higher dose (and in rats).