"Tryptophan (abbreviated as Trp or W) is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in genetic code as the codon UGG. ....
     "For many organisms (including humans), tryptophan is an essential amino acid. This means that it cannot be synthesized by the organism and therefore must be part of its diet. Amino acids, including tryptophan, act as building blocks in protein biosynthesis. In addition, tryptophan functions as a biochemical precursor for the following compounds:
    * Serotonin (a neurotransmitter), synthesized via tryptophan hydroxylase.... Serotonin, in turn, can be converted to melatonin (a neurohormone) ... 
    * Niacin ¹

     "The amino acid tryptophan is a safe, non-addicting sleeping aid which works because it is made into serotonin in the brain.  Serotonin is the neurotransmitter which intitiates sleep." ²

     "Serotonin is a neurotransmitter which has important inhibitory functions in the brain, to prevent excess nervous stimulation at night so that sleep can occur.  It is also a growth hormone releaser. ...
     "What can be done about ... sleep problems?  One way is to increase the amount of serotonin in the brain by taking the essential amino acid tryptophan (found in milk and bananas [and turkey meat] for example).  That hot cup of milk (nonfat is just as good) before bedtime really can help you sleep.    Tryptophan is a basic building block that the body can convert to serotonin.  Increasing serotonin levels usually improves ease of falling asleep and may reduce the problem of waking up in the night.  Tryptophan is much more effective used as the pure material taken on an empty stomach because amino acids in food compete for transport into the brain.  Two grams [2,000mg] of tryptophan taken at, not before, bedtime is a typical adult dose.  Both vitamins B-6 (100 milligrams) and C [1,000mg], [and the nutrients magnesium (200 milligrams) and omega 3 fatty acids] are required for the conversion of tryptophan to serotonin [all of which are nutrients commonly deficient in American diets].  Excess serotonin can cause vascular headaches, nasal congestion, or, more rarely, constipation [too little serotonin can cause migraine headaches, insomnia, severe mental disturbances and bizarre, even violent, behavior]." ³

     "Serotonin affects many basic psychological functions such as anxiety mechanisms and the regulation of mood, thoughts, aggression, appetite, sex drive and the sleep/wake cycle.  Multiple observations suggest that serotonin ... plays an important role in the regulation of mood and a key role in the treatment of depression. 
     "Due to this, lack of serotonin is often associated with depression. Restoring the normal or enhanced level of this neurotransmitter acts as mood enhancer. Prozac is a mood enhancing drug, which acts in the central nervous system by inhibiting the re-uptake mechanism of serotonin into the synapse. The synapse is a junction where a signal is transmitted from one nerve cell to another. Since serotonin is not degraded in the synaptic cleft, Prozac [as well as other SSRIs or Selective Serotonin Re-uptake Inhibitors] promotes a prolonged presence of serotonin keeping the post-synaptic membrane active [these are prescribed on either the assumption or the proof that a high enough level of serotonin is already available in the patient's system.  If it's not it may prove to be counter-productive, exaggerating the symptoms associated with low serotonin levels].
     "In the central nervous system, serotonin is involved in fear and 'flight' responses, an activity which is opposed by the aggression or 'fight' stimulating hormone adrenaline (epinephrine) and neurotransmitter dopamine. " ⁴

     "Melatonin, 5-methoxy-N-acetyltryptamine, is a naturally occurring hormone found in most animals, including humans, and some other living organisms, including algae.  Circulating levels vary in a daily cycle, and melatonin plays a role in the regulation of the circadian rhythms of several biological functions. Many biological effects of melatonin are produced through activation of melatonin receptors, while others are due to its role as a pervasive and powerful antioxidant with a particular role in the protection of nuclear and mitochondrial DNA
     "The use of melatonin as a drug can entrain the circadian clock to environmental cycles and can have beneficial effects for treatment of certain insomnias. Its therapeutic potential may be limited by its short biological half-life, poor bioavailability, and the fact that it has numerous non-specific actions.
     "Products containing melatonin have been available as a dietary supplement in the United States since 1993,[7] and met with good consumer acceptance and enthusiasm. ...
     "In higher animals and humans, melatonin is produced by pinealocytes in the pineal gland (located in the brain) and also by the retina, lens and GI tract. It is naturally synthesized from the amino acid tryptophan (via synthesis of serotonin) by the enzyme 5-hydroxyindole-O-methyltransferase. ...
     "Melatonin is also synthesized by various plants, such as rice, and ingested melatonin has been shown to be capable of reaching and binding to melatonin binding sites in the brains of mammals. ...
     "During the night, melatonin regulates leptin [and other energy factors], lowering the levels." ⁵

     "Melatonin ... is considered to be the ultimate route towards an uninterrupted sleep. It also controls other vital bodily functions, including the metabolism rate and ones sex drive. Perhaps most excitingly however, melatonin is thought to hold the key to the door behind which the prospect of human life extension lies. The idea of being able to extend the human lifespan makes melatonin one of the most widely researched topics today. ...
     "Melatonin is the hormone that helps to regulate sleep-wake cycles, and can be found within all living creatures. Besides this key role, melatonin is thought to have many other useful properties, and minimal side effects. As a result, it is available as an over the counter drug, even though it is secreted naturally within humans. While the natural form can be contaminated by viruses, the synthetic product is often used to eliminate sleeping disorders and to minimise the effects of jet lag. Older people may find it particularly useful because as humans age, the amount of melatonin secreted in the body decreases. This gives some explanation as to why young people find it easier to sleep than older people." ⁶

¹ Wikipedia,http://en.wikipedia.org/wiki/Tryptophan 5/19/08
² Pearson and Shaw, Life Extension: A Practical Scientific Approach, New York: Warner Books, Inc., 1983, p.179
³ Pearson and Shaw, Life Extension: A Practical Scientific Approach, New York: Warner Books, Inc., 1983, p.192
⁴ Imperial College, London: Department of Chemistry, http://www.ch.ic.ac.uk/wiki/index.php/It:Serotonin 5/19/08
⁵ Wikipedia, http://en.wikipedia.org/wiki/Melatonin 5/19/08
⁶ Imperial College, London: Department of Chemistry, http://www.ch.ic.ac.uk/wiki/index.php/It:Melatonin 5/19/08