I will let the sources do the talking:

Quote Originally Posted by Thor
Just to follow up a bit on this. I did a search on Google, and I found this article: "Biogenic Amines and the States of Sleep", Michel Jouvet, Science 163 (862) pages : 32-41 (1969). A couple of relevant quotes from the article (my emphasis):
"In birds, whose blood brain barrier is permeable, intravenous injection of serotonin or noradrenalin leads to slow-wave sleep, as indicated by hehavior and by electroencephalographic recording (25)."
"The increase of cerebral serotonin (through intravenous or intraperitoneal injection of its precursor 5-hydroxytryptophan (5 HTP)) leads first to a state which resembles slow-wave sleep (28). However, this drug leads to a suppression of paradoxical sleep for 5 to 6 hours; the suppression is followed by a secondary rebound (29)"
Note that "paradoxical sleep" is better known today as REM sleep.
Quote Originally Posted by Thor
Here is the abstract of a scientific paper, "5-Hydroxytryptophan: a clinically-effective serotonin precursor" by Birdsall TC [email protected] Altern Med Rev 1998 Aug; 3(4):271-80. (Emphasis is mine.):
5-Hydroxytryptophan (5-HTP) is the intermediate metabolite of the essential amino acid L-tryptophan (LT) in the biosynthesis of serotonin. Intestinal absorption of 5-HTP does not require the presence of a transport molecule, and is not affected by the presence of other amino acids; therefore it may be taken with meals without reducing its effectiveness. Unlike LT, 5-HTP cannot be shunted into niacin or protein production. Therapeutic use of 5-HTP bypasses the conversion of LT into 5-HTP by the enzyme tryptophan hydroxylase, which is the rate-limiting step in the synthesis of serotonin. 5-HTP is well absorbed from an oral dose, with about 70 percent ending up in the bloodstream. It easily crosses the blood-brain barrier and effectively increases central nervous system (CNS) synthesis of serotonin. In the CNS, serotonin levels have been implicated in the regulation of sleep, depression, anxiety, aggression, appetite, temperature, sexual behaviour, and pain sensation. Therapeutic administration of 5-HTP has been shown to be effective in treating a wide variety of conditions, including depression, fibromyalgia, binge eating associated with obesity, chronic headaches, and insomnia.
REM Sleep–like Atonia of Hypoglossal (XII) Motoneurons Is Caused by Loss of Noradrenergic and Serotonergic Inputs
http://ajrccm.atsjournals.org/cgi/re...72/10/1322.pdf
+ Loss of serotonin input causes REM sleep.

REM sleep and depression: common neurobiological control mechanisms
http://ajp.psychiatryonline.org/cgi/...ract/139/5/565
+ Serotonin intake suppresses REM sleep. (I thought I recalled this study.. hence why I retracted my earlier statement.)

Experiments in the Chemistry of Normal Sleep
http://bjp.rcpsych.org/cgi/content/a...urcetype=HWCIT
+ It is postulated that the effect on sleep results from the metabolism of tryptophan to 5-hydroxytryptamine (serotonin).

What do you think..? I hope this has been enlightening.
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