The pineal gland is what converts tryptophan (amino acid, from foods) to, essentially, melatonin. And marijuana raises melatonin 4,000%, in two hours (Lissoni, P., Resentini, M., and Fraschini, F. "Effects of Tetrahydrocannabinol on Melatonin Secretion in Man." Hormone and Metabolic Research 1986; 77-78. At baseline, the mean value of their melatonin levels was 21.3 pg/ml. Two hours later, it was 904 pg/ml.), so it's stimulating the pineal gland greatly, as to how, I don't know.

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