Maybe that's why I always sleep better when I finish off the evening with a glass of milk… (Tryptophan is the amino acid in Turkey that ensures Thanksgiving Day is quiet after lunch. Maybe.)

From To-Done.com's article Tips for getting to sleep faster & sleeping better

Drink milk. Milk has an amino acid in it called Tryptophan that increase the levels of serotonin and/or melatonin in the brain which slow down brain activity. It's science folks.

[Thanks to Lifehacker for the link.]